tview/textarea.go

package tview

import (
	"strings"
	"unicode"
	"unicode/utf8"

	"github.com/gdamore/tcell/v2"
	"github.com/rivo/uniseg"
)

const (
	// The minimum capacity of the text area's piece chain slice.
	pieceChainMinCap = 10

	// The minimum capacity of the text area's edit buffer.
	editBufferMinCap = 200

	// The maximum number of bytes making up a grapheme cluster. In theory, this
	// could be longer but it would be highly unusual.
	maxGraphemeClusterSize = 40

	// The minimum width of text (if available) to be shown left of the cursor.
	minCursorPrefix = 5

	// The minimum width of text (if available) to be shown right of the cursor.
	minCursorSuffix = 3
)

// Types of user actions on a text area.
type taAction int

const (
	taActionOther        taAction = iota
	taActionTypeSpace             // Typing a space character.
	taActionTypeNonSpace          // Typing a non-space character.
	taActionBackspace             // Deleting the previous character.
	taActionDelete                // Deleting the next character.
)

// NewLine is the string sequence to be inserted when hitting the Enter key in a
// TextArea. The default is "\n" but you may change it to "\r\n" if required.
var NewLine = "\n"

// textAreaSpan represents a range of text in a text area. The text area widget
// roughly follows the concept of Piece Chains outlined in
// http://www.catch22.net/tuts/neatpad/piece-chains with some modifications.
// This type represents a "span" (or "piece") and thus refers to a subset of the
// text in the editor as part of a doubly-linked list.
//
// In most places where we reference a position in the text, we use a
// three-element int array. The first element is the index of the referenced
// span in the piece chain. The second element is the offset into the span's
// referenced text (relative to the span's start), its value is always >= 0 and
// < span.length. The third element is the state of the text parser at that
// position.
//
// A range of text is represented by a span range which is a starting position
// (3-int array) and an ending position (3-int array). The starting position
// references the first character of the range, the ending position references
// the position after the last character of the range. The end of the text is
// therefore always [3]int{1, 0, 0}, position 0 of the ending sentinel.
//
// Sentinel spans are dummy spans not referring to any text. There are always
// two sentinel spans: the starting span at index 0 of the [TextArea.spans]
// slice and the ending span at index 1.
type textAreaSpan struct {
	// Links to the previous and next textAreaSpan objects as indices into the
	// [TextArea.spans] slice. The sentinel spans (index 0 and 1) have -1 as
	// their previous or next links, respectively.
	previous, next int

	// The start index and the length of the text segment this span represents.
	// If "length" is negative, the span represents a substring of
	// [TextArea.initialText] and the actual length is its absolute value. If it
	// is positive, the span represents a substring of [TextArea.editText]. For
	// the sentinel spans (index 0 and 1), both values will be 0. Others will
	// never have a zero length.
	offset, length int
}

// textAreaUndoItem represents an undoable edit to the text area. It describes
// the two spans wrapping a text change.
type textAreaUndoItem struct {
	before, after                 int    // The index of the copied "before" and "after" spans into the "spans" slice.
	originalBefore, originalAfter int    // The original indices of the "before" and "after" spans.
	pos                           [3]int // The cursor position to be assumed after applying an undo.
	length                        int    // The total text length at the time the undo item was created.
	continuation                  bool   // If true, this item is a continuation of the previous undo item. It is handled together with all other undo items in the same continuation sequence.
}

// TextArea implements a simple text editor for multi-line text. Multi-color
// text is not supported. Word-wrapping is enabled by default but can be turned
// off or be changed to character-wrapping.
//
// At this point, a text area cannot be added to a [Form]. This will be added in
// the future.
//
// # Navigation and Editing
//
// A text area is always in editing mode and no other mode exists. The following
// keys can be used to move the cursor (subject to what the user's terminal
// supports and how it is configured):
//
//   - Left arrow: Move left.
//   - Right arrow: Move right.
//   - Down arrow: Move down.
//   - Up arrow: Move up.
//   - Ctrl-A, Home: Move to the beginning of the current line.
//   - Ctrl-E, End: Move to the end of the current line.
//   - Ctrl-F, page down: Move down by one page.
//   - Ctrl-B, page up: Move up by one page.
//   - Alt-Up arrow: Scroll the page up, leaving the cursor in its position.
//   - Alt-Down arrow: Scroll the page down, leaving the cursor in its position.
//   - Alt-Left arrow: Scroll the page to the left, leaving the cursor in its
//     position. Ignored if wrapping is enabled.
//   - Alt-Right arrow:  Scroll the page to the right, leaving the cursor in its
//     position. Ignored if wrapping is enabled.
//   - Alt-B, Ctrl-Left arrow: Jump to the beginning of the current or previous
//     word.
//   - Alt-F, Ctrl-Right arrow: Jump to the end of the current or next word.
//
// Words are defined according to [Unicode Standard Annex #29]. We skip any
// words that contain only spaces or punctuation.
//
// Entering a character will insert it at the current cursor location.
// Subsequent characters are shifted accordingly. If the cursor is outside the
// visible area, any changes to the text will move it into the visible area. The
// following keys can also be used to modify the text:
//
//   - Enter: Insert a newline character (see [NewLine]).
//   - Tab: Insert a tab character (\t). It will be rendered like [TabSize]
//     spaces. (This may eventually be changed to behave like regular tabs.)
//   - Ctrl-H, Backspace: Delete one character to the left of the cursor.
//   - Ctrl-D, Delete: Delete the character under the cursor (or the first
//     character on the next line if the cursor is at the end of a line).
//   - Alt-Backspace: Delete the word to the left of the cursor.
//   - Ctrl-K: Delete everything under and to the right of the cursor until the
//     next newline character.
//   - Ctrl-W: Delete from the start of the current word to the left of the
//     cursor.
//   - Ctrl-U: Delete the current line, i.e. everything after the last newline
//     character before the cursor up until the next newline character. This may
//     span multiple visible rows if wrapping is enabled.
//
// Text can be selected by moving the cursor while holding the Shift key, to the
// extent that this is supported by the user's terminal. The Ctrl-L key can be
// used to select the entire text. (Ctrl-A already binds to the "Home" key.)
//
// When text is selected:
//
//   - Entering a character will replace the selected text with the new
//     character.
//   - Backspace, delete, Ctrl-H, Ctrl-D: Delete the selected text.
//   - Ctrl-Q: Copy the selected text into the clipboard, unselect the text.
//   - Ctrl-X: Copy the selected text into the clipboard and delete it.
//   - Ctrl-V: Replace the selected text with the clipboard text. If no text is
//     selected, the clipboard text will be inserted at the cursor location.
//
// The Ctrl-Q key was chosen for the "copy" function because the Ctrl-C key is
// the default key to stop the application. If your application frees up the
// global Ctrl-C key and you want to bind it to the "copy to clipboard"
// function, you may use [Box.SetInputCapture] to override the Ctrl-Q key to
// implement copying to the clipboard. Note that using your terminal's /
// operating system's key bindings for copy+paste functionality may not have the
// expected effect as tview will not be able to handle these keys. Pasting text
// using your operating system's or terminal's own methods may be very slow as
// each character will be pasted individually.
//
// The default clipboard is an internal text buffer, i.e. the operating system's
// clipboard is not used. If you want to implement your own clipboard (or make
// use of your operating system's clipboard), you can use
// [TextArea.SetClipboard] which  provides all the functionality needed to
// implement your own clipboard.
//
// The text area also supports Undo:
//
//   - Ctrl-Z: Undo the last change.
//   - Ctrl-Y: Redo the last Undo change.
//
// Undo does not affect the clipboard.
//
// If the mouse is enabled, the following actions are available:
//
//   - Left click: Move the cursor to the clicked position or to the end of the
//     line if past the last character.
//   - Left double-click: Select the word under the cursor.
//   - Left click while holding the Shift key: Select text.
//   - Scroll wheel: Scroll the text.
//
// [Unicode Standard Annex #29]: https://unicode.org/reports/tr29/
type TextArea struct {
	*Box

	// Whether or not this text area is disabled/read-only.
	disabled bool

	// The size of the text area. If set to 0, the text area will use the entire
	// available space.
	width, height int

	// The text to be shown in the text area when it is empty.
	placeholder string

	// The label text shown, usually when part of a form.
	label string

	// The width of the text area's label.
	labelWidth int

	// Styles:

	// The label style.
	labelStyle tcell.Style

	// The style of the text. Background colors different from the Box's
	// background color may lead to unwanted artefacts.
	textStyle tcell.Style

	// The style of the selected text.
	selectedStyle tcell.Style

	// The style of the placeholder text.
	placeholderStyle tcell.Style

	// Text manipulation related fields:

	// The text area's text prior to any editing. It is referenced by spans with
	// a negative length.
	initialText string

	// Any text that's been added by the user at some point. We only ever append
	// to this buffer. It is referenced by spans with a positive length.
	editText strings.Builder

	// The total length of all text in the text area.
	length int

	// The maximum number of bytes allowed in the text area. If 0, there is no
	// limit.
	maxLength int

	// The piece chain. The first two spans are sentinel spans which don't
	// reference anything and always remain in the same place. Spans are never
	// deleted from this slice.
	spans []textAreaSpan

	// Display, navigation, and cursor related fields:

	// If set to true, lines that are longer than the available width are
	// wrapped onto the next line. If set to false, any characters beyond the
	// available width are discarded.
	wrap bool

	// If set to true and if wrap is also true, lines are split at spaces or
	// after punctuation characters.
	wordWrap bool

	// The index of the first line shown in the text area.
	rowOffset int

	// The number of cells to be skipped on each line (not used in wrap mode).
	columnOffset int

	// The inner height and width of the text area the last time it was drawn.
	lastHeight, lastWidth int

	// The width of the currently known widest line, as determined by
	// [TextArea.extendLines].
	widestLine int

	// Text positions and states of the start of lines. Each element is a span
	// position (see [textAreaSpan]). Not all lines of the text may be contained
	// at any time, extend as needed with the [TextArea.extendLines] function.
	lineStarts [][3]int

	// The cursor always points to the next position where a new character would
	// be placed. The selection start is the same as cursor as long as there is
	// no selection. When there is one, the selection is between selectionStart
	// and cursor.
	cursor, selectionStart struct {
		// The row and column in screen space but relative to the start of the
		// text which may be outside the text area's box. The column value may
		// be larger than where the cursor actually is if the line the cursor
		// is on is shorter. The actualColumn is the position as it is seen on
		// screen. These three values may not be determined yet, in which case
		// the row is negative.
		row, column, actualColumn int

		// The textAreaSpan position with state for the actual next character.
		pos [3]int
	}

	// Set to true when the mouse is dragging to select text.
	dragging bool

	// Clipboard related fields:

	// The internal clipboard.
	clipboard string

	// The function to call when the user copies/cuts a text selection to the
	// clipboard.
	copyToClipboard func(string)

	// The function to call when the user pastes text from the clipboard.
	pasteFromClipboard func() string

	// Undo/redo related fields:

	// The last action performed by the user.
	lastAction taAction

	// The undo stack's items. Each item is a copy of the span before the
	// modified span range and a copy of the span after the modified span range.
	// To undo an action, the two referenced spans are put back into their
	// original place. Undos and redos decrease or increase the nextUndo value.
	// Thus, the next undo action is not always the last item.
	undoStack []textAreaUndoItem

	// The current undo/redo position on the undo stack. If no undo or redo has
	// been performed yet, this is the same as len(undoStack).
	nextUndo int

	// Event handlers:

	// An optional function which is called when the input has changed.
	changed func()

	// An optional function which is called when the position of the cursor or
	// the selection has changed.
	moved func()

	// A callback function set by the Form class and called when the user leaves
	// this form item.
	finished func(tcell.Key)
}

// NewTextArea returns a new text area. Use [TextArea.SetText] to set the
// initial text.
func NewTextArea() *TextArea {
	t := &TextArea{
		Box:              NewBox(),
		wrap:             true,
		wordWrap:         true,
		placeholderStyle: tcell.StyleDefault.Background(Styles.PrimitiveBackgroundColor).Foreground(Styles.TertiaryTextColor),
		labelStyle:       tcell.StyleDefault.Foreground(Styles.SecondaryTextColor),
		textStyle:        tcell.StyleDefault.Background(Styles.PrimitiveBackgroundColor).Foreground(Styles.PrimaryTextColor),
		selectedStyle:    tcell.StyleDefault.Background(Styles.PrimaryTextColor).Foreground(Styles.PrimitiveBackgroundColor),
		spans:            make([]textAreaSpan, 2, pieceChainMinCap), // We reserve some space to avoid reallocations right when editing starts.
		lastAction:       taActionOther,
	}
	t.editText.Grow(editBufferMinCap)
	t.spans[0] = textAreaSpan{previous: -1, next: 1}
	t.spans[1] = textAreaSpan{previous: 0, next: -1}
	t.cursor.pos = [3]int{1, 0, -1}
	t.selectionStart = t.cursor
	t.SetClipboard(nil, nil)

	return t
}

// SetText sets the text of the text area. All existing text is deleted and
// replaced with the new text. Any edits are discarded, no undos are available.
// This function is typically only used to initialize the text area with a text
// after it has been created. To clear the text area's text (again, no undos),
// provide an empty string.
//
// If cursorAtTheEnd is false, the cursor is placed at the start of the text. If
// it is true, it is placed at the end of the text. For very long texts, placing
// the cursor at the end can be an expensive operation because the entire text
// needs to be parsed and laid out.
//
// If you want to set text and preserve undo functionality, use
// [TextArea.Replace] instead.
func (t *TextArea) SetText(text string, cursorAtTheEnd bool) *TextArea {
	t.spans = t.spans[:2]
	t.initialText = text
	t.editText.Reset()
	t.lineStarts = nil
	t.length = len(text)
	t.rowOffset = 0
	t.columnOffset = 0
	t.reset()
	t.cursor.row, t.cursor.actualColumn, t.cursor.column = 0, 0, 0
	t.cursor.pos = [3]int{1, 0, -1}
	t.undoStack = t.undoStack[:0]

	if len(text) > 0 {
		t.spans = append(t.spans, textAreaSpan{
			previous: 0,
			next:     1,
			offset:   0,
			length:   -len(text),
		})
		t.spans[0].next = 2
		t.spans[1].previous = 2
		if cursorAtTheEnd {
			t.cursor.row = -1
			if t.lastWidth > 0 {
				t.findCursor(true, 0)
			}
		} else {
			t.cursor.pos = [3]int{2, 0, -1}
		}
	} else {
		t.spans[0].next = 1
		t.spans[1].previous = 0
	}
	t.selectionStart = t.cursor

	if t.changed != nil {
		t.changed()
	}

	if t.lastWidth > 0 && t.moved != nil {
		t.moved()
	}

	return t
}

// GetText returns the entire text of the text area. Note that this will newly
// allocate the entire text.
func (t *TextArea) GetText() string {
	if t.length == 0 {
		return ""
	}

	var text strings.Builder
	text.Grow(t.length)
	spanIndex := t.spans[0].next
	for spanIndex != 1 {
		span := &t.spans[spanIndex]
		if span.length < 0 {
			text.WriteString(t.initialText[span.offset : span.offset-span.length])
		} else {
			text.WriteString(t.editText.String()[span.offset : span.offset+span.length])
		}
		spanIndex = t.spans[spanIndex].next
	}

	return text.String()
}

// HasSelection returns whether the selected text is non-empty.
func (t *TextArea) HasSelection() bool {
	return t.selectionStart != t.cursor
}

// GetSelection returns the currently selected text and its start and end
// positions within the entire text as a half-open interval. If the returned
// text is an empty string, the start and end positions are the same and can be
// interpreted as the cursor position.
//
// Calling this function will result in string allocations as well as a search
// for text positions. This is expensive if the text has been edited extensively
// already. Use [TextArea.HasSelection] first if you are only interested in
// selected text.
func (t *TextArea) GetSelection() (text string, start int, end int) {
	from, to := t.selectionStart.pos, t.cursor.pos
	if t.cursor.row < t.selectionStart.row || (t.cursor.row == t.selectionStart.row && t.cursor.actualColumn < t.selectionStart.actualColumn) {
		from, to = to, from
	}

	if from[0] == 1 {
		start = t.length
	}
	if to[0] == 1 {
		end = t.length
	}

	var (
		index     int
		selection strings.Builder
		inside    bool
	)
	for span := t.spans[0].next; span != 1; span = t.spans[span].next {
		var spanText string
		length := t.spans[span].length
		if length < 0 {
			length = -length
			spanText = t.initialText
		} else {
			spanText = t.editText.String()
		}
		spanText = spanText[t.spans[span].offset : t.spans[span].offset+length]

		if from[0] == span && to[0] == span {
			if from != to {
				selection.WriteString(spanText[from[1]:to[1]])
			}
			start = index + from[1]
			end = index + to[1]
			break
		} else if from[0] == span {
			if from != to {
				selection.WriteString(spanText[from[1]:])
			}
			start = index + from[1]
			inside = true
		} else if to[0] == span {
			if from != to {
				selection.WriteString(spanText[:to[1]])
			}
			end = index + to[1]
			break
		} else if inside && from != to {
			selection.WriteString(spanText)
		}

		index += length
	}

	if selection.Len() != 0 {
		text = selection.String()
	}
	return
}

// GetCursor returns the current cursor position where the first character of
// the entire text is in row 0, column 0. If the user has selected text, the
// "from" values will refer to the beginning of the selection and the "to"
// values to the end of the selection (exclusive). They are the same if there
// is no selection.
func (t *TextArea) GetCursor() (fromRow, fromColumn, toRow, toColumn int) {
	fromRow, fromColumn = t.selectionStart.row, t.selectionStart.actualColumn
	toRow, toColumn = t.cursor.row, t.cursor.actualColumn
	if toRow < fromRow || (toRow == fromRow && toColumn < fromColumn) {
		fromRow, fromColumn, toRow, toColumn = toRow, toColumn, fromRow, fromColumn
	}
	if t.length > 0 && t.wrap && fromColumn >= t.lastWidth { // This happens when a row has text all the way until the end, pushing the cursor outside the viewport.
		fromRow++
		fromColumn = 0
	}
	if t.length > 0 && t.wrap && toColumn >= t.lastWidth {
		toRow++
		toColumn = 0
	}
	return
}

// GetTextLength returns the string length of the text in the text area.
func (t *TextArea) GetTextLength() int {
	return t.length
}

// Replace replaces a section of the text with new text. The start and end
// positions refer to index positions within the entire text string (as a
// half-open interval). They may be the same, in which case text is inserted at
// the given position. If the text is an empty string, text between start and
// end is deleted. Index positions will be shifted to line up with character
// boundaries.
//
// Previous selections are cleared. The cursor will be located at the end of the
// replaced text. Scroll offsets will not be changed.
//
// The effects of this function can be undone (and redone) by the user.
func (t *TextArea) Replace(start, end int, text string) *TextArea {
	t.Select(start, end)
	row := t.selectionStart.row
	t.cursor.pos = t.replace(t.selectionStart.pos, t.cursor.pos, text, false)
	t.cursor.row = -1
	t.truncateLines(row - 1)
	t.findCursor(false, row)
	t.selectionStart = t.cursor
	if t.changed != nil {
		t.changed()
	}
	if t.moved != nil {
		t.moved()
	}
	return t
}

// Select selects a section of the text. The start and end positions refer to
// index positions within the entire text string (as a half-open interval). They
// may be the same, in which case the cursor is placed at the given position.
// Any previous selection is removed. Scroll offsets will be preserved.
//
// Index positions will be shifted to line up with character boundaries.
func (t *TextArea) Select(start, end int) *TextArea {
	oldFrom, oldTo := t.selectionStart, t.cursor
	defer func() {
		if (oldFrom != t.selectionStart || oldTo != t.cursor) && t.moved != nil {
			t.moved()
		}
	}()

	// Clamp input values.
	if start < 0 {
		start = 0
	}
	if start > t.length {
		start = t.length
	}
	if end < 0 {
		end = 0
	}
	if end > t.length {
		end = t.length
	}
	if end < start {
		start, end = end, start
	}

	// Find the cursor positions.
	var row, index int
	t.cursor.row, t.cursor.pos = -1, [3]int{1, 0, -1}
	t.selectionStart = t.cursor
RowLoop:
	for {
		if row >= len(t.lineStarts) {
			t.extendLines(t.lastWidth, row)
			if row >= len(t.lineStarts) {
				break
			}
		}

		// Check the spans of this row.
		pos := t.lineStarts[row]
		var (
			next      [3]int
			lineIndex int
		)
		if row+1 < len(t.lineStarts) {
			next = t.lineStarts[row+1]
		} else {
			next = [3]int{1, 0, -1}
		}
		for {
			if pos[0] == next[0] {
				if start >= index+lineIndex && start < index+lineIndex+next[1]-pos[1] ||
					end >= index+lineIndex && end < index+lineIndex+next[1]-pos[1] {
					break
				}
				index += lineIndex + next[1] - pos[1]
				row++
				continue RowLoop // Move on to the next row.
			} else {
				length := t.spans[pos[0]].length
				if length < 0 {
					length = -length
				}
				if start >= index+lineIndex && start < index+lineIndex+length-pos[1] ||
					end >= index+lineIndex && end < index+lineIndex+length-pos[1] {
					break
				}
				lineIndex += length - pos[1]
				pos[0], pos[1] = t.spans[pos[0]].next, 0
			}
		}

		// One of the indices is in this row. Step through it.
		pos = t.lineStarts[row]
		endPos := pos
		var (
			cluster, text string
			column, width int
		)
		for pos != next {
			if t.selectionStart.row < 0 && start <= index {
				t.selectionStart.row, t.selectionStart.column, t.selectionStart.actualColumn = row, column, column
				t.selectionStart.pos = pos
			}
			if t.cursor.row < 0 && end <= index {
				t.cursor.row, t.cursor.column, t.cursor.actualColumn = row, column, column
				t.cursor.pos = pos
				break RowLoop
			}
			cluster, text, _, width, pos, endPos = t.step(text, pos, endPos)
			index += len(cluster)
			column += width
		}
	}

	if t.cursor.row < 0 {
		t.findCursor(false, 0) // This only happens if we couldn't find the locations above.
		t.selectionStart = t.cursor
	}

	return t
}

// SetWrap sets the flag that, if true, leads to lines that are longer than the
// available width being wrapped onto the next line. If false, any characters
// beyond the available width are not displayed.
func (t *TextArea) SetWrap(wrap bool) *TextArea {
	if t.wrap != wrap {
		t.wrap = wrap
		t.reset()
	}
	return t
}

// SetWordWrap sets the flag that causes lines that are longer than the
// available width to be wrapped onto the next line at spaces or after
// punctuation marks (according to [Unicode Standard Annex #14]). This flag is
// ignored if the flag set with [TextArea.SetWrap] is false. The text area's
// default is word-wrapping.
//
// [Unicode Standard Annex #14]: https://www.unicode.org/reports/tr14/
func (t *TextArea) SetWordWrap(wrapOnWords bool) *TextArea {
	if t.wordWrap != wrapOnWords {
		t.wordWrap = wrapOnWords
		t.reset()
	}
	return t
}

// SetPlaceholder sets the text to be displayed when the text area is empty.
func (t *TextArea) SetPlaceholder(placeholder string) *TextArea {
	t.placeholder = placeholder
	return t
}

// SetLabel sets the text to be displayed before the text area.
func (t *TextArea) SetLabel(label string) *TextArea {
	t.label = label
	return t
}

// GetLabel returns the text to be displayed before the text area.
func (t *TextArea) GetLabel() string {
	return t.label
}

// SetLabelWidth sets the screen width of the label. A value of 0 will cause the
// primitive to use the width of the label string.
func (t *TextArea) SetLabelWidth(width int) *TextArea {
	t.labelWidth = width
	return t
}

// SetSize sets the screen size of the input element of the text area. The input
// element is always located next to the label which is always located in the
// top left corner. If any of the values are 0 or larger than the available
// space, the available space will be used.
func (t *TextArea) SetSize(rows, columns int) *TextArea {
	t.width = columns
	t.height = rows
	return t
}

// GetFieldWidth returns this primitive's field width.
func (t *TextArea) GetFieldWidth() int {
	return t.width
}

// GetFieldHeight returns this primitive's field height.
func (t *TextArea) GetFieldHeight() int {
	return t.height
}

// SetDisabled sets whether or not the item is disabled / read-only.
func (t *TextArea) SetDisabled(disabled bool) FormItem {
	t.disabled = disabled
	if t.finished != nil {
		t.finished(-1)
	}
	return t
}

// SetMaxLength sets the maximum number of bytes allowed in the text area. A
// value of 0 means there is no limit. If the text area currently contains more
// bytes than this, it may violate this constraint.
func (t *TextArea) SetMaxLength(maxLength int) *TextArea {
	t.maxLength = maxLength
	return t
}

// SetLabelStyle sets the style of the label.
func (t *TextArea) SetLabelStyle(style tcell.Style) *TextArea {
	t.labelStyle = style
	return t
}

// GetLabelStyle returns the style of the label.
func (t *TextArea) GetLabelStyle() tcell.Style {
	return t.labelStyle
}

// SetTextStyle sets the style of the text. Background colors different from the
// Box's background color may lead to unwanted artefacts.
func (t *TextArea) SetTextStyle(style tcell.Style) *TextArea {
	t.textStyle = style
	return t
}

// SetSelectedStyle sets the style of the selected text.
func (t *TextArea) SetSelectedStyle(style tcell.Style) *TextArea {
	t.selectedStyle = style
	return t
}

// SetPlaceholderStyle sets the style of the placeholder text.
func (t *TextArea) SetPlaceholderStyle(style tcell.Style) *TextArea {
	t.placeholderStyle = style
	return t
}

// GetOffset returns the text's offset, that is, the number of rows and columns
// skipped during drawing at the top or on the left, respectively. Note that the
// column offset is ignored if wrapping is enabled.
func (t *TextArea) GetOffset() (row, column int) {
	return t.rowOffset, t.columnOffset
}

// SetOffset sets the text's offset, that is, the number of rows and columns
// skipped during drawing at the top or on the left, respectively. If wrapping
// is enabled, the column offset is ignored. These values may get adjusted
// automatically to ensure that some text is always visible.
func (t *TextArea) SetOffset(row, column int) *TextArea {
	t.rowOffset, t.columnOffset = row, column
	return t
}

// SetClipboard allows you to implement your own clipboard by providing a
// function that is called when the user wishes to store text in the clipboard
// (copyToClipboard) and a function that is called when the user wishes to
// retrieve text from the clipboard (pasteFromClipboard).
//
// Providing nil values will cause the default clipboard implementation to be
// used.
func (t *TextArea) SetClipboard(copyToClipboard func(string), pasteFromClipboard func() string) *TextArea {
	t.copyToClipboard = copyToClipboard
	if t.copyToClipboard == nil {
		t.copyToClipboard = func(text string) {
			t.clipboard = text
		}
	}

	t.pasteFromClipboard = pasteFromClipboard
	if t.pasteFromClipboard == nil {
		t.pasteFromClipboard = func() string {
			return t.clipboard
		}
	}

	return t
}

// SetChangedFunc sets a handler which is called whenever the text of the text
// area has changed.
func (t *TextArea) SetChangedFunc(handler func()) *TextArea {
	t.changed = handler
	return t
}

// SetMovedFunc sets a handler which is called whenever the cursor position or
// the text selection has changed.
func (t *TextArea) SetMovedFunc(handler func()) *TextArea {
	t.moved = handler
	return t
}

// SetFinishedFunc sets a callback invoked when the user leaves this form item.
func (t *TextArea) SetFinishedFunc(handler func(key tcell.Key)) FormItem {
	t.finished = handler
	return t
}

// Focus is called when this primitive receives focus.
func (t *TextArea) Focus(delegate func(p Primitive)) {
	// If we're part of a form and this item is disabled, there's nothing the
	// user can do here so we're finished.
	if t.finished != nil && t.disabled {
		t.finished(-1)
		return
	}

	t.Box.Focus(delegate)
}

// SetFormAttributes sets attributes shared by all form items.
func (t *TextArea) SetFormAttributes(labelWidth int, labelColor, bgColor, fieldTextColor, fieldBgColor tcell.Color) FormItem {
	t.labelWidth = labelWidth
	t.backgroundColor = bgColor
	t.labelStyle = t.labelStyle.Foreground(labelColor)
	t.textStyle = tcell.StyleDefault.Foreground(fieldTextColor).Background(fieldBgColor)
	return t
}

// replace deletes a range of text and inserts the given text at that position.
// If the resulting text would exceed the maximum length, the function does not
// do anything. The function returns the end position of the deleted/inserted
// range. The provided row is the row of the deleted range start.
//
// The function can hang if "deleteStart" is located after "deleteEnd".
//
// Undo events are always generated unless continuation is true and text is
// either appended to the end of a span or a span is shortened at the beginning
// or the end (and nothing else).
//
// This function does not modify [TextArea.lineStarts].
func (t *TextArea) replace(deleteStart, deleteEnd [3]int, insert string, continuation bool) [3]int {
	// Maybe nothing needs to be done?
	if deleteStart == deleteEnd && insert == "" || t.maxLength > 0 && len(insert) > 0 && t.length+len(insert) >= t.maxLength {
		return deleteEnd
	}

	// Notify at the end.
	if t.changed != nil {
		defer t.changed()
	}

	// Handle a few cases where we don't put anything onto the undo stack for
	// increased efficiency.
	if continuation {
		// Same action as the one before. An undo item was already generated for
		// this block of (same) actions. We're also only changing one character.
		switch {
		case insert == "" && deleteStart[1] != 0 && deleteEnd[1] == 0:
			// Simple backspace. Just shorten this span.
			length := t.spans[deleteStart[0]].length
			if length < 0 {
				t.length -= -length - deleteStart[1]
				length = -deleteStart[1]
			} else {
				t.length -= length - deleteStart[1]
				length = deleteStart[1]
			}
			t.spans[deleteStart[0]].length = length
			return deleteEnd
		case insert == "" && deleteStart[1] == 0 && deleteEnd[1] != 0:
			// Simple delete. Just clip the beginning of this span.
			t.spans[deleteEnd[0]].offset += deleteEnd[1]
			if t.spans[deleteEnd[0]].length < 0 {
				t.spans[deleteEnd[0]].length += deleteEnd[1]
			} else {
				t.spans[deleteEnd[0]].length -= deleteEnd[1]
			}
			t.length -= deleteEnd[1]
			deleteEnd[1] = 0
			return deleteEnd
		case insert != "" && deleteStart == deleteEnd && deleteEnd[1] == 0:
			previous := t.spans[deleteStart[0]].previous
			bufferSpan := t.spans[previous]
			if bufferSpan.length > 0 && bufferSpan.offset+bufferSpan.length == t.editText.Len() {
				// Typing individual characters. Simply extend the edit buffer.
				length, _ := t.editText.WriteString(insert)
				t.spans[previous].length += length
				t.length += length
				return deleteEnd
			}
		}
	}

	// All other cases generate an undo item.
	before := t.spans[deleteStart[0]].previous
	after := deleteEnd[0]
	if deleteEnd[1] > 0 {
		after = t.spans[deleteEnd[0]].next
	}
	t.undoStack = t.undoStack[:t.nextUndo]
	t.undoStack = append(t.undoStack, textAreaUndoItem{
		before:         len(t.spans),
		after:          len(t.spans) + 1,
		originalBefore: before,
		originalAfter:  after,
		length:         t.length,
		pos:            t.cursor.pos,
		continuation:   continuation,
	})
	t.spans = append(t.spans, t.spans[before])
	t.spans = append(t.spans, t.spans[after])
	t.nextUndo++

	// Adjust total text length by subtracting everything between "before" and
	// "after". Inserted spans will be added back.
	for index := deleteStart[0]; index != after; index = t.spans[index].next {
		if t.spans[index].length < 0 {
			t.length += t.spans[index].length
		} else {
			t.length -= t.spans[index].length
		}
	}
	t.spans[before].next = after
	t.spans[after].previous = before

	// We go from left to right, connecting new spans as needed. We update
	// "before" as the span to connect new spans to.

	// If we start deleting in the middle of a span, connect a partial span.
	if deleteStart[1] != 0 {
		span := textAreaSpan{
			previous: before,
			next:     after,
			offset:   t.spans[deleteStart[0]].offset,
			length:   deleteStart[1],
		}
		if t.spans[deleteStart[0]].length < 0 {
			span.length = -span.length
		}
		t.length += deleteStart[1] // This was previously subtracted.
		t.spans[before].next = len(t.spans)
		t.spans[after].previous = len(t.spans)
		before = len(t.spans)
		for row, lineStart := range t.lineStarts { // Also redirect line starts until the end of this new span.
			if lineStart[0] == deleteStart[0] {
				if lineStart[1] >= deleteStart[1] {
					t.lineStarts = t.lineStarts[:row] // Everything else is unknown at this point.
					break
				}
				t.lineStarts[row][0] = len(t.spans)
			}
		}
		t.spans = append(t.spans, span)
	}

	// If we insert text, connect a new span.
	if insert != "" {
		span := textAreaSpan{
			previous: before,
			next:     after,
			offset:   t.editText.Len(),
		}
		span.length, _ = t.editText.WriteString(insert)
		t.length += span.length
		t.spans[before].next = len(t.spans)
		t.spans[after].previous = len(t.spans)
		before = len(t.spans)
		t.spans = append(t.spans, span)
	}

	// If we stop deleting in the middle of a span, connect a partial span.
	if deleteEnd[1] != 0 {
		span := textAreaSpan{
			previous: before,
			next:     after,
			offset:   t.spans[deleteEnd[0]].offset + deleteEnd[1],
		}
		length := t.spans[deleteEnd[0]].length
		if length < 0 {
			span.length = length + deleteEnd[1]
			t.length -= span.length // This was previously subtracted.
		} else {
			span.length = length - deleteEnd[1]
			t.length += span.length // This was previously subtracted.
		}
		t.spans[before].next = len(t.spans)
		t.spans[after].previous = len(t.spans)
		deleteEnd[0], deleteEnd[1] = len(t.spans), 0
		t.spans = append(t.spans, span)
	}

	return deleteEnd
}

// Draw draws this primitive onto the screen.
func (t *TextArea) Draw(screen tcell.Screen) {
	t.Box.DrawForSubclass(screen, t)

	// Prepare
	x, y, width, height := t.GetInnerRect()
	if width <= 0 || height <= 0 {
		return // We have no space for anything.
	}
	columnOffset := t.columnOffset
	if t.wrap {
		columnOffset = 0
	}

	// Draw label.
	_, labelBg, _ := t.labelStyle.Decompose()
	if t.labelWidth > 0 {
		labelWidth := t.labelWidth
		if labelWidth > width {
			labelWidth = width
		}
		printWithStyle(screen, t.label, x, y, 0, labelWidth, AlignLeft, t.labelStyle, labelBg == tcell.ColorDefault)
		x += labelWidth
		width -= labelWidth
	} else {
		_, drawnWidth, _, _ := printWithStyle(screen, t.label, x, y, 0, width, AlignLeft, t.labelStyle, labelBg == tcell.ColorDefault)
		x += drawnWidth
		width -= drawnWidth
	}

	// What's the space for the input element?
	if t.width > 0 && t.width < width {
		width = t.width
	}
	if t.height > 0 && t.height < height {
		height = t.height
	}
	if width <= 0 {
		return // No space left for the text area.
	}

	// Draw the input element if necessary.
	_, bg, _ := t.textStyle.Decompose()
	if t.disabled {
		bg = t.backgroundColor
	}
	if bg != t.backgroundColor {
		for row := 0; row < height; row++ {
			for column := 0; column < width; column++ {
				screen.SetContent(x+column, y+row, ' ', nil, t.textStyle)
			}
		}
	}

	// Show/hide the cursor at the end.
	defer func() {
		if t.HasFocus() {
			row, column := t.cursor.row, t.cursor.actualColumn
			if t.length > 0 && t.wrap && column >= t.lastWidth { // This happens when a row has text all the way until the end, pushing the cursor outside the viewport.
				row++
				column = 0
			}
			if row >= 0 &&
				row-t.rowOffset >= 0 && row-t.rowOffset < height &&
				column-columnOffset >= 0 && column-columnOffset < width {
				screen.ShowCursor(x+column-columnOffset, y+row-t.rowOffset)
			} else {
				screen.HideCursor()
			}
		}
	}()

	// Placeholder.
	if t.length == 0 && len(t.placeholder) > 0 {
		t.drawPlaceholder(screen, x, y, width, height)
		return // We're done already.
	}

	// Make sure the visible lines are broken over.
	firstDrawing := t.lastWidth == 0
	if t.lastWidth != width && t.lineStarts != nil {
		t.reset()
	}
	t.lastHeight, t.lastWidth = height, width
	t.extendLines(width, t.rowOffset+height)
	if len(t.lineStarts) <= t.rowOffset {
		return // It's scrolled out of view.
	}

	// If the cursor position is unknown, find it. This usually only happens
	// before the screen is drawn for the first time.
	if t.cursor.row < 0 {
		t.findCursor(true, 0)
		if t.selectionStart.row < 0 {
			t.selectionStart = t.cursor
		}
		if firstDrawing && t.moved != nil {
			t.moved()
		}
	}

	// Print the text.
	var cluster, text string
	line := t.rowOffset
	pos := t.lineStarts[line]
	endPos := pos
	posX, posY := 0, 0
	for pos[0] != 1 {
		var clusterWidth int
		cluster, text, _, clusterWidth, pos, endPos = t.step(text, pos, endPos)

		// Prepare drawing.
		runes := []rune(cluster)
		style := t.selectedStyle
		fromRow, fromColumn := t.cursor.row, t.cursor.actualColumn
		toRow, toColumn := t.selectionStart.row, t.selectionStart.actualColumn
		if fromRow > toRow || fromRow == toRow && fromColumn > toColumn {
			fromRow, fromColumn, toRow, toColumn = toRow, toColumn, fromRow, fromColumn
		}
		if toRow < line ||
			toRow == line && toColumn <= posX ||
			fromRow > line ||
			fromRow == line && fromColumn > posX {
			style = t.textStyle
			if t.disabled {
				style = style.Background(t.backgroundColor)
			}
		}

		// Draw character.
		if posX+clusterWidth-columnOffset <= width && posX-columnOffset >= 0 && clusterWidth > 0 {
			screen.SetContent(x+posX-columnOffset, y+posY, runes[0], runes[1:], style)
		}

		// Advance.
		posX += clusterWidth
		if line+1 < len(t.lineStarts) && t.lineStarts[line+1] == pos {
			// We must break over.
			posY++
			if posY >= height {
				break // Done.
			}
			posX = 0
			line++
		}
	}
}

// drawPlaceholder draws the placeholder text into the given rectangle. It does
// not do anything if the text area already contains text or if there is no
// placeholder text.
func (t *TextArea) drawPlaceholder(screen tcell.Screen, x, y, width, height int) {
	posX, posY := x, y
	lastLineBreak, lastGraphemeBreak := x, x // Screen positions of the last possible line/grapheme break.
	iterateString(t.placeholder, func(main rune, comb []rune, textPos, textWidth, screenPos, screenWidth, boundaries int) bool {
		if posX+screenWidth > x+width {
			// This character doesn't fit. Break over to the next line.
			// Perform word wrapping first by copying the last word over to
			// the next line.
			clearX := lastLineBreak
			if lastLineBreak == x {
				clearX = lastGraphemeBreak
			}
			posY++
			if posY >= y+height {
				return true
			}
			newPosX := x
			for clearX < posX {
				main, comb, _, _ := screen.GetContent(clearX, posY-1)
				screen.SetContent(clearX, posY-1, ' ', nil, tcell.StyleDefault.Background(t.backgroundColor))
				screen.SetContent(newPosX, posY, main, comb, t.placeholderStyle)
				clearX++
				newPosX++
			}
			lastLineBreak, lastGraphemeBreak, posX = x, x, newPosX
		}

		// Draw this character.
		screen.SetContent(posX, posY, main, comb, t.placeholderStyle)
		posX += screenWidth
		switch boundaries & uniseg.MaskLine {
		case uniseg.LineMustBreak:
			posY++
			if posY >= y+height {
				return true
			}
			posX = x
		case uniseg.LineCanBreak:
			lastLineBreak = posX
		}
		lastGraphemeBreak = posX

		return false
	})
}

// reset resets many of the local variables of the text area because they cannot
// be used anymore and must be recalculated, typically after the text area's
// size has changed.
func (t *TextArea) reset() {
	t.truncateLines(0)
	if t.wrap {
		t.cursor.row = -1
		t.selectionStart.row = -1
	}
	t.widestLine = 0
}

// extendLines traverses the current text and extends [TextArea.lineStarts] such
// that it describes at least maxLines+1 lines (or less if the text is shorter).
// Text is laid out for the given width while respecting the wrapping settings.
// It is assumed that if [TextArea.lineStarts] already has entries, they obey
// the same rules.
//
// If width is 0, nothing happens.
func (t *TextArea) extendLines(width, maxLines int) {
	if width <= 0 {
		return
	}

	// Start with the first span.
	if len(t.lineStarts) == 0 {
		if len(t.spans) > 2 {
			t.lineStarts = append(t.lineStarts, [3]int{t.spans[0].next, 0, -1})
		} else {
			return // No text.
		}
	}

	// Determine starting positions and starting spans.
	pos := t.lineStarts[len(t.lineStarts)-1] // The starting position is the last known line.
	endPos := pos
	var (
		cluster, text                       string
		lineWidth, clusterWidth, boundaries int
		lastGraphemeBreak, lastLineBreak    [3]int
		widthSinceLineBreak                 int
	)
	for pos[0] != 1 {
		// Get the next grapheme cluster.
		cluster, text, boundaries, clusterWidth, pos, endPos = t.step(text, pos, endPos)
		lineWidth += clusterWidth
		widthSinceLineBreak += clusterWidth

		// Any line breaks?
		if !t.wrap || lineWidth <= width {
			if boundaries&uniseg.MaskLine == uniseg.LineMustBreak && (len(text) > 0 || uniseg.HasTrailingLineBreakInString(cluster)) {
				// We must break over.
				t.lineStarts = append(t.lineStarts, pos)
				if lineWidth > t.widestLine {
					t.widestLine = lineWidth
				}
				lineWidth = 0
				lastGraphemeBreak = [3]int{}
				lastLineBreak = [3]int{}
				widthSinceLineBreak = 0
				if len(t.lineStarts) > maxLines {
					break // We have enough lines, we can stop.
				}
				continue
			}
		} else { // t.wrap && lineWidth > width
			if !t.wordWrap || lastLineBreak == [3]int{} {
				if lastGraphemeBreak != [3]int{} { // We have at least one character on each line.
					// Break after last grapheme.
					t.lineStarts = append(t.lineStarts, lastGraphemeBreak)
					if lineWidth > t.widestLine {
						t.widestLine = lineWidth
					}
					lineWidth = clusterWidth
					lastLineBreak = [3]int{}
				}
			} else { // t.wordWrap && lastLineBreak != [3]int{}
				// Break after last line break opportunity.
				t.lineStarts = append(t.lineStarts, lastLineBreak)
				if lineWidth > t.widestLine {
					t.widestLine = lineWidth
				}
				lineWidth = widthSinceLineBreak
				lastLineBreak = [3]int{}
			}
		}

		// Analyze break opportunities.
		if boundaries&uniseg.MaskLine == uniseg.LineCanBreak {
			lastLineBreak = pos
			widthSinceLineBreak = 0
		}
		lastGraphemeBreak = pos

		// Can we stop?
		if len(t.lineStarts) > maxLines {
			break
		}
	}
}

// truncateLines truncates the trailing lines of the [TextArea.lineStarts]
// slice such that len(lineStarts) <= fromLine. If fromLine is negative, a value
// of 0 is assumed. If it is greater than the length of lineStarts, nothing
// happens.
func (t *TextArea) truncateLines(fromLine int) {
	if fromLine < 0 {
		fromLine = 0
	}
	if fromLine < len(t.lineStarts) {
		t.lineStarts = t.lineStarts[:fromLine]
	}
}

// findCursor determines the cursor position if its "row" value is < 0
// (=unknown) but only its span position ("pos" value) is known. If the cursor
// position is already known (row >= 0), it can also be used to modify row and
// column offsets such that the cursor is visible during the next call to
// [TextArea.Draw], by setting "clamp" to true.
//
// To determine the cursor position, "startRow" helps reduce processing time by
// indicating the lowest row in which searching should start. Set this to 0 if
// you don't have any information where the cursor might be (but know that this
// is expensive for long texts).
//
// The cursor's desired column will be set to its actual column.
func (t *TextArea) findCursor(clamp bool, startRow int) {
	defer func() {
		t.cursor.column = t.cursor.actualColumn
	}()

	if !clamp && t.cursor.row >= 0 {
		return // Nothing to do.
	}

	// Clamp to viewport.
	if clamp && t.cursor.row >= 0 {
		cursorRow := t.cursor.row
		if t.wrap && t.cursor.actualColumn >= t.lastWidth {
			cursorRow++ // A row can push the cursor just outside the viewport. It will wrap onto the next line.
		}
		if cursorRow < t.rowOffset {
			// We're above the viewport.
			t.rowOffset = cursorRow
		} else if cursorRow >= t.rowOffset+t.lastHeight {
			// We're below the viewport.
			t.rowOffset = cursorRow - t.lastHeight + 1
			if t.rowOffset >= len(t.lineStarts) {
				t.extendLines(t.lastWidth, t.rowOffset)
				if t.rowOffset >= len(t.lineStarts) {
					t.rowOffset = len(t.lineStarts) - 1
					if t.rowOffset < 0 {
						t.rowOffset = 0
					}
				}
			}
		}
		if !t.wrap {
			if t.cursor.actualColumn < t.columnOffset+minCursorPrefix {
				// We're left of the viewport.
				t.columnOffset = t.cursor.actualColumn - minCursorPrefix
				if t.columnOffset < 0 {
					t.columnOffset = 0
				}
			} else if t.cursor.actualColumn >= t.columnOffset+t.lastWidth-minCursorSuffix {
				// We're right of the viewport.
				t.columnOffset = t.cursor.actualColumn - t.lastWidth + minCursorSuffix
				if t.columnOffset >= t.widestLine {
					t.columnOffset = t.widestLine - 1
					if t.columnOffset < 0 {
						t.columnOffset = 0
					}
				}
			}
		}
		return
	}

	// The screen position of the cursor is unknown. Find it. This can be
	// expensive. First, find the row.
	row := startRow
	if row < 0 {
		row = 0
	}
RowLoop:
	for {
		// Examine the current row.
		if row+1 >= len(t.lineStarts) {
			t.extendLines(t.lastWidth, row+1)
		}
		if row >= len(t.lineStarts) {
			t.cursor.row, t.cursor.actualColumn, t.cursor.pos = row, 0, [3]int{1, 0, -1}
			break // It's the end of the text.
		}

		// Check this row's spans to see if the cursor is in this row.
		pos := t.lineStarts[row]
		for pos[0] != 1 {
			if row+1 >= len(t.lineStarts) {
				break // It's the last row so the cursor must be in this row.
			}
			if t.cursor.pos[0] == pos[0] {
				// The cursor is in this span.
				if t.lineStarts[row+1][0] == pos[0] {
					// The next row starts with the same span.
					if t.cursor.pos[1] >= t.lineStarts[row+1][1] {
						// The cursor is not in this row.
						row++
						continue RowLoop
					} else {
						// The cursor is in this row.
						break
					}
				} else {
					// The next row starts with a different span. The cursor
					// must be in this row.
					break
				}
			} else {
				// The cursor is in a different span.
				if t.lineStarts[row+1][0] == pos[0] {
					// The next row starts with the same span. This row is
					// irrelevant.
					row++
					continue RowLoop
				} else {
					// The next row starts with a different span. Move towards it.
					pos = [3]int{t.spans[pos[0]].next, 0, -1}
				}
			}
		}

		// Try to find the screen position in this row.
		pos = t.lineStarts[row]
		endPos := pos
		column := 0
		var text string
		for {
			if pos[0] == 1 || t.cursor.pos[0] == pos[0] && t.cursor.pos[1] == pos[1] {
				// We found the position. We're done.
				t.cursor.row, t.cursor.actualColumn, t.cursor.pos = row, column, pos
				break RowLoop
			}
			var clusterWidth int
			_, text, _, clusterWidth, pos, endPos = t.step(text, pos, endPos)
			if row+1 < len(t.lineStarts) && t.lineStarts[row+1] == pos {
				// We reached the end of the line. Go to the next one.
				row++
				continue RowLoop
			}
			column += clusterWidth
		}
	}

	if clamp && t.cursor.row >= 0 {
		// We know the position now. Adapt offsets.
		t.findCursor(true, startRow)
	}
}

// step is similar to [github.com/rivo/uniseg.StepString] but it iterates over
// the piece chain, starting with "pos", a span position plus state (which may
// be -1 for the start of the text). The returned "boundaries" value is same
// value returned by [github.com/rivo/uniseg.StepString], "width" is the screen
// width of the grapheme. The "pos" and "endPos" positions refer to the start
// and the end of the "text" string, respectively. For the first call, text may
// be empty and pos/endPos may be the same. For consecutive calls, provide
// "rest" as the text and "newPos" and "newEndPos" as the new positions/states.
// An empty "rest" string indicates the end of the text. The "endPos" state is
// irrelevant.
func (t *TextArea) step(text string, pos, endPos [3]int) (cluster, rest string, boundaries, width int, newPos, newEndPos [3]int) {
	if pos[0] == 1 {
		return // We're already past the end.
	}

	// We want to make sure we have a text at least the size of a grapheme
	// cluster.
	span := t.spans[pos[0]]
	if len(text) < maxGraphemeClusterSize &&
		(span.length < 0 && -span.length-pos[1] >= maxGraphemeClusterSize ||
			span.length > 0 && t.spans[pos[0]].length-pos[1] >= maxGraphemeClusterSize) {
		// We can use a substring of one span.
		if span.length < 0 {
			text = t.initialText[span.offset+pos[1] : span.offset-span.length]
		} else {
			text = t.editText.String()[span.offset+pos[1] : span.offset+span.length]
		}
		endPos = [3]int{span.next, 0, -1}
	} else {
		// We have to compose the text from multiple spans.
		for len(text) < maxGraphemeClusterSize && endPos[0] != 1 {
			endSpan := t.spans[endPos[0]]
			var moreText string
			if endSpan.length < 0 {
				moreText = t.initialText[endSpan.offset+endPos[1] : endSpan.offset-endSpan.length]
			} else {
				moreText = t.editText.String()[endSpan.offset+endPos[1] : endSpan.offset+endSpan.length]
			}
			if len(moreText) > maxGraphemeClusterSize {
				moreText = moreText[:maxGraphemeClusterSize]
			}
			text += moreText
			endPos[1] += len(moreText)
			if endPos[1] >= endSpan.length {
				endPos[0], endPos[1] = endSpan.next, 0
			}
		}
	}

	// Run the grapheme cluster iterator.
	cluster, text, boundaries, pos[2] = uniseg.StepString(text, pos[2])
	pos[1] += len(cluster)
	for pos[0] != 1 && (span.length < 0 && pos[1] >= -span.length || span.length >= 0 && pos[1] >= span.length) {
		pos[0] = span.next
		if span.length < 0 {
			pos[1] += span.length
		} else {
			pos[1] -= span.length
		}
		span = t.spans[pos[0]]
	}

	if cluster == "\t" {
		width = TabSize
	} else {
		width = boundaries >> uniseg.ShiftWidth
	}

	return cluster, text, boundaries, width, pos, endPos
}

// moveCursor sets the cursor's screen position and span position for the given
// row and column which are screen space coordinates relative to the top-left
// corner of the text area's full text (visible or not). The column value may be
// negative, in which case, the cursor will be placed at the end of the line.
// The cursor's actual position will be aligned with a grapheme cluster
// boundary. The next call to [TextArea.Draw] will attempt to keep the cursor in
// the viewport.
func (t *TextArea) moveCursor(row, column int) {
	// Are we within the range of rows?
	if len(t.lineStarts) <= row {
		// No. Extent the line buffer.
		t.extendLines(t.lastWidth, row)
	}
	if len(t.lineStarts) == 0 {
		return // No lines. Nothing to do.
	}
	if row < 0 {
		// We're at the start of the text.
		row = 0
		column = 0
	} else if row >= len(t.lineStarts) {
		// We're already past the end.
		row = len(t.lineStarts) - 1
		column = -1
	}

	// Iterate through this row until we find the position.
	t.cursor.row, t.cursor.actualColumn = row, 0
	if t.wrap {
		t.cursor.actualColumn = 0
	}
	pos := t.lineStarts[row]
	endPos := pos
	var text string
	for pos[0] != 1 {
		var clusterWidth int
		oldPos := pos // We may have to revert to this position.
		_, text, _, clusterWidth, pos, endPos = t.step(text, pos, endPos)
		if len(t.lineStarts) > row+1 && pos == t.lineStarts[row+1] || // We've reached the end of the line.
			column >= 0 && t.cursor.actualColumn+clusterWidth > column { // We're past the requested column.
			pos = oldPos
			break
		}
		t.cursor.actualColumn += clusterWidth
	}

	if column < 0 {
		t.cursor.column = t.cursor.actualColumn
	} else {
		t.cursor.column = column
	}
	t.cursor.pos = pos
	t.findCursor(true, row)
}

// moveWordRight moves the cursor to the end of the current or next word. If
// after is set to true, the cursor will be placed after the word. If false, the
// cursor will be placed on the last character of the word. If clamp is set to
// true, the cursor will be visible during the next call to [TextArea.Draw].
func (t *TextArea) moveWordRight(after, clamp bool) {
	// Because we rely on clampToCursor to calculate the new screen position,
	// this is an expensive operation for large texts.
	pos := t.cursor.pos
	endPos := pos
	var (
		cluster, text string
		inWord        bool
	)
	for pos[0] != 0 {
		var boundaries int
		oldPos := pos
		cluster, text, boundaries, _, pos, endPos = t.step(text, pos, endPos)
		if oldPos == t.cursor.pos {
			continue // Skip the first character.
		}
		firstRune, _ := utf8.DecodeRuneInString(cluster)
		if !unicode.IsSpace(firstRune) && !unicode.IsPunct(firstRune) {
			inWord = true
		}
		if inWord && boundaries&uniseg.MaskWord != 0 {
			if !after {
				pos = oldPos
			}
			break
		}
	}
	startRow := t.cursor.row
	t.cursor.row, t.cursor.column, t.cursor.actualColumn = -1, 0, 0
	t.cursor.pos = pos
	t.findCursor(clamp, startRow)
}

// moveWordLeft moves the cursor to the beginning of the current or previous
// word. If clamp is true, the cursor will be visible during the next call to
// [TextArea.Draw].
func (t *TextArea) moveWordLeft(clamp bool) {
	// We go back row by row, trying to find the last word boundary before the
	// cursor.
	row := t.cursor.row
	if row+1 < len(t.lineStarts) {
		t.extendLines(t.lastWidth, row+1)
	}
	if row >= len(t.lineStarts) {
		row = len(t.lineStarts) - 1
	}
	for row >= 0 {
		pos := t.lineStarts[row]
		endPos := pos
		var lastWordBoundary [3]int
		var (
			cluster, text string
			inWord        bool
			boundaries    int
		)
		for pos[0] != 1 && pos != t.cursor.pos {
			oldBoundaries := boundaries
			oldPos := pos
			cluster, text, boundaries, _, pos, endPos = t.step(text, pos, endPos)
			firstRune, _ := utf8.DecodeRuneInString(cluster)
			wordRune := !unicode.IsSpace(firstRune) && !unicode.IsPunct(firstRune)
			if oldBoundaries&uniseg.MaskWord != 0 {
				if pos != t.cursor.pos && !inWord && wordRune {
					// A boundary transitioning from a space/punctuation word to
					// a letter word.
					lastWordBoundary = oldPos
				}
				inWord = false
			}
			if wordRune {
				inWord = true
			}
		}
		if lastWordBoundary[0] != 0 {
			// We found something.
			t.cursor.pos = lastWordBoundary
			break
		}
		row--
	}
	if row < 0 {
		// We didn't find anything. We're at the start of the text.
		t.cursor.pos = [3]int{t.spans[0].next, 0, -1}
		row = 0
	}
	t.cursor.row, t.cursor.column, t.cursor.actualColumn = -1, 0, 0
	t.findCursor(clamp, row)
}

// deleteLine deletes all characters between the last newline before the cursor
// and the next newline after the cursor (inclusive).
func (t *TextArea) deleteLine() {
	// We go back row by row, trying to find the last mandatory line break
	// before the cursor.
	startRow := t.cursor.row
	if t.cursor.actualColumn == 0 && t.cursor.pos[0] == 1 {
		startRow-- // If we're at the very end, delete the row before.
	}
	if startRow+1 < len(t.lineStarts) {
		t.extendLines(t.lastWidth, startRow+1)
	}
	if len(t.lineStarts) == 0 {
		return // Nothing to delete.
	}
	if startRow >= len(t.lineStarts) {
		startRow = len(t.lineStarts) - 1
	}
	for startRow >= 0 {
		// What's the last rune before the start of the line?
		pos := t.lineStarts[startRow]
		span := t.spans[pos[0]]
		var text string
		if pos[1] > 0 {
			// Extract text from this span.
			if span.length < 0 {
				text = t.initialText
			} else {
				text = t.editText.String()
			}
			text = text[:span.offset+pos[1]]
		} else {
			// Extract text from the previous span.
			if span.previous != 0 {
				span = t.spans[span.previous]
				if span.length < 0 {
					text = t.initialText[:span.offset-span.length]
				} else {
					text = t.editText.String()[:span.offset+span.length]
				}
			}
		}
		if uniseg.HasTrailingLineBreakInString(text) {
			// The row before this one ends with a mandatory line break. This is
			// the first line we will delete.
			break
		}
		startRow--
	}
	if startRow < 0 {
		// We didn't find anything. It'll be the first line.
		startRow = 0
	}

	// Find the next line break after the cursor.
	pos := t.cursor.pos
	endPos := pos
	var cluster, text string
	for pos[0] != 1 {
		cluster, text, _, _, pos, endPos = t.step(text, pos, endPos)
		if uniseg.HasTrailingLineBreakInString(cluster) {
			break
		}
	}

	// Delete the text.
	t.cursor.pos = t.replace(t.lineStarts[startRow], pos, "", false)
	t.cursor.row = -1
	t.truncateLines(startRow)
	t.findCursor(true, startRow)
}

// getSelection returns the current selection as span locations where the first
// returned location is always before or the same as the second returned
// location. This assumes that the cursor and selection positions are known. The
// third return value is the starting row of the selection.
func (t *TextArea) getSelection() ([3]int, [3]int, int) {
	from := t.selectionStart.pos
	to := t.cursor.pos
	row := t.selectionStart.row
	if t.cursor.row < t.selectionStart.row ||
		(t.cursor.row == t.selectionStart.row && t.cursor.actualColumn < t.selectionStart.actualColumn) {
		from, to = to, from
		row = t.cursor.row
	}
	return from, to, row
}

// getSelectedText returns the text of the current selection.
func (t *TextArea) getSelectedText() string {
	var text strings.Builder

	from, to, _ := t.getSelection()
	for from[0] != to[0] {
		span := t.spans[from[0]]
		if span.length < 0 {
			text.WriteString(t.initialText[span.offset+from[1] : span.offset-span.length])
		} else {
			text.WriteString(t.editText.String()[span.offset+from[1] : span.offset+span.length])
		}
		from[0], from[1] = span.next, 0
	}
	if from[0] != 1 && from[1] < to[1] {
		span := t.spans[from[0]]
		if span.length < 0 {
			text.WriteString(t.initialText[span.offset+from[1] : span.offset+to[1]])
		} else {
			text.WriteString(t.editText.String()[span.offset+from[1] : span.offset+to[1]])
		}
	}

	return text.String()
}

// InputHandler returns the handler for this primitive.
func (t *TextArea) InputHandler() func(event *tcell.EventKey, setFocus func(p Primitive)) {
	return t.WrapInputHandler(func(event *tcell.EventKey, setFocus func(p Primitive)) {
		if t.disabled {
			return
		}

		// All actions except a few specific ones are "other" actions.
		newLastAction := taActionOther
		defer func() {
			t.lastAction = newLastAction
		}()

		// Trigger a "moved" event if requested.
		if t.moved != nil {
			selectionStart, cursor := t.selectionStart, t.cursor
			defer func() {
				if selectionStart != t.selectionStart || cursor != t.cursor {
					t.moved()
				}
			}()
		}

		// Process the different key events.
		switch key := event.Key(); key {
		case tcell.KeyLeft: // Move one grapheme cluster to the left.
			if event.Modifiers()&tcell.ModAlt == 0 {
				// Regular movement.
				if event.Modifiers()&tcell.ModShift == 0 && t.selectionStart.pos != t.cursor.pos {
					// Move to the start of the selection.
					if t.selectionStart.row < t.cursor.row || (t.selectionStart.row == t.cursor.row && t.selectionStart.actualColumn < t.cursor.actualColumn) {
						t.cursor = t.selectionStart
					}
					t.findCursor(true, t.cursor.row)
				} else if event.Modifiers()&tcell.ModMeta != 0 || event.Modifiers()&tcell.ModCtrl != 0 {
					// This captures Ctrl-Left on some systems.
					t.moveWordLeft(event.Modifiers()&tcell.ModShift != 0)
				} else if t.cursor.actualColumn == 0 {
					// Move to the end of the previous row.
					if t.cursor.row > 0 {
						t.moveCursor(t.cursor.row-1, -1)
					}
				} else {
					// Move one grapheme cluster to the left.
					t.moveCursor(t.cursor.row, t.cursor.actualColumn-1)
				}
				if event.Modifiers()&tcell.ModShift == 0 {
					t.selectionStart = t.cursor
				}
			} else if !t.wrap { // This doesn't work on all terminals.
				// Just scroll.
				t.columnOffset--
				if t.columnOffset < 0 {
					t.columnOffset = 0
				}
			}
		case tcell.KeyRight: // Move one grapheme cluster to the right.
			if event.Modifiers()&tcell.ModAlt == 0 {
				// Regular movement.
				if event.Modifiers()&tcell.ModShift == 0 && t.selectionStart.pos != t.cursor.pos {
					// Move to the end of the selection.
					if t.selectionStart.row > t.cursor.row || (t.selectionStart.row == t.cursor.row && t.selectionStart.actualColumn > t.cursor.actualColumn) {
						t.cursor = t.selectionStart
					}
					t.findCursor(true, t.cursor.row)
				} else if t.cursor.pos[0] != 1 {
					if event.Modifiers()&tcell.ModMeta != 0 || event.Modifiers()&tcell.ModCtrl != 0 {
						// This captures Ctrl-Right on some systems.
						t.moveWordRight(event.Modifiers()&tcell.ModShift != 0, true)
					} else {
						// Move one grapheme cluster to the right.
						var clusterWidth int
						_, _, _, clusterWidth, t.cursor.pos, _ = t.step("", t.cursor.pos, t.cursor.pos)
						if len(t.lineStarts) <= t.cursor.row+1 {
							t.extendLines(t.lastWidth, t.cursor.row+1)
						}
						if t.cursor.row+1 < len(t.lineStarts) && t.lineStarts[t.cursor.row+1] == t.cursor.pos {
							// We've reached the end of the line.
							t.cursor.row++
							t.cursor.actualColumn = 0
							t.cursor.column = 0
							t.findCursor(true, t.cursor.row)
						} else {
							// Move one character to the right.
							t.moveCursor(t.cursor.row, t.cursor.actualColumn+clusterWidth)
						}
					}
				}
				if event.Modifiers()&tcell.ModShift == 0 {
					t.selectionStart = t.cursor
				}
			} else if !t.wrap { // This doesn't work on all terminals.
				// Just scroll.
				t.columnOffset++
				if t.columnOffset >= t.widestLine {
					t.columnOffset = t.widestLine - 1
					if t.columnOffset < 0 {
						t.columnOffset = 0
					}
				}
			}
		case tcell.KeyDown: // Move one row down.
			if event.Modifiers()&tcell.ModAlt == 0 {
				// Regular movement.
				column := t.cursor.column
				t.moveCursor(t.cursor.row+1, t.cursor.column)
				t.cursor.column = column
				if event.Modifiers()&tcell.ModShift == 0 {
					t.selectionStart = t.cursor
				}
			} else {
				// Just scroll.
				t.rowOffset++
				if t.rowOffset >= len(t.lineStarts) {
					t.extendLines(t.lastWidth, t.rowOffset)
					if t.rowOffset >= len(t.lineStarts) {
						t.rowOffset = len(t.lineStarts) - 1
						if t.rowOffset < 0 {
							t.rowOffset = 0
						}
					}
				}
			}
		case tcell.KeyUp: // Move one row up.
			if event.Modifiers()&tcell.ModAlt == 0 {
				// Regular movement.
				column := t.cursor.column
				t.moveCursor(t.cursor.row-1, t.cursor.column)
				t.cursor.column = column
				if event.Modifiers()&tcell.ModShift == 0 {
					t.selectionStart = t.cursor
				}
			} else {
				// Just scroll.
				t.rowOffset--
				if t.rowOffset < 0 {
					t.rowOffset = 0
				}
			}
		case tcell.KeyHome, tcell.KeyCtrlA: // Move to the start of the line.
			t.moveCursor(t.cursor.row, 0)
			if event.Modifiers()&tcell.ModShift == 0 {
				t.selectionStart = t.cursor
			}
		case tcell.KeyEnd, tcell.KeyCtrlE: // Move to the end of the line.
			t.moveCursor(t.cursor.row, -1)
			if event.Modifiers()&tcell.ModShift == 0 {
				t.selectionStart = t.cursor
			}
		case tcell.KeyPgDn, tcell.KeyCtrlF: // Move one page down.
			column := t.cursor.column
			t.moveCursor(t.cursor.row+t.lastHeight, t.cursor.column)
			t.cursor.column = column
			if event.Modifiers()&tcell.ModShift == 0 {
				t.selectionStart = t.cursor
			}
		case tcell.KeyPgUp, tcell.KeyCtrlB: // Move one page up.
			column := t.cursor.column
			t.moveCursor(t.cursor.row-t.lastHeight, t.cursor.column)
			t.cursor.column = column
			if event.Modifiers()&tcell.ModShift == 0 {
				t.selectionStart = t.cursor
			}
		case tcell.KeyEnter: // Insert a newline.
			from, to, row := t.getSelection()
			t.cursor.pos = t.replace(from, to, NewLine, t.lastAction == taActionTypeSpace)
			t.cursor.row = -1
			t.truncateLines(row - 1)
			t.findCursor(true, row)
			t.selectionStart = t.cursor
			newLastAction = taActionTypeSpace
		case tcell.KeyTab: // Insert a tab character. It will be rendered as TabSize spaces.
			// But forwarding takes precedence.
			if t.finished != nil {
				t.finished(key)
				return
			}

			from, to, row := t.getSelection()
			t.cursor.pos = t.replace(from, to, "\t", t.lastAction == taActionTypeSpace)
			t.cursor.row = -1
			t.truncateLines(row - 1)
			t.findCursor(true, row)
			t.selectionStart = t.cursor
			newLastAction = taActionTypeSpace
		case tcell.KeyBacktab, tcell.KeyEscape: // Only used in forms.
			if t.finished != nil {
				t.finished(key)
				return
			}
		case tcell.KeyRune:
			if event.Modifiers()&tcell.ModAlt > 0 {
				// We accept some Alt- key combinations.
				switch event.Rune() {
				case 'f':
					if event.Modifiers()&tcell.ModShift == 0 {
						t.moveWordRight(false, true)
						t.selectionStart = t.cursor
					} else {
						t.moveWordRight(true, true)
					}
				case 'b':
					t.moveWordLeft(true)
					if event.Modifiers()&tcell.ModShift == 0 {
						t.selectionStart = t.cursor
					}
				}
			} else {
				// Other keys are simply accepted as regular characters.
				r := event.Rune()
				from, to, row := t.getSelection()
				newLastAction = taActionTypeNonSpace
				if unicode.IsSpace(r) {
					newLastAction = taActionTypeSpace
				}
				t.cursor.pos = t.replace(from, to, string(r), newLastAction == t.lastAction || t.lastAction == taActionTypeNonSpace && newLastAction == taActionTypeSpace)
				t.cursor.row = -1
				t.truncateLines(row - 1)
				t.findCursor(true, row)
				t.selectionStart = t.cursor
			}
		case tcell.KeyBackspace, tcell.KeyBackspace2: // Delete backwards. tcell.KeyBackspace is the same as tcell.CtrlH.
			from, to, row := t.getSelection()
			if from != to {
				// Simply delete the current selection.
				t.cursor.pos = t.replace(from, to, "", false)
				t.cursor.row = -1
				t.truncateLines(row - 1)
				t.findCursor(true, row)
				t.selectionStart = t.cursor
				break
			}

			beforeCursor := t.cursor
			if event.Modifiers()&tcell.ModAlt == 0 {
				// Move the cursor back by one grapheme cluster.
				if t.cursor.actualColumn == 0 {
					// Move to the end of the previous row.
					if t.cursor.row > 0 {
						t.moveCursor(t.cursor.row-1, -1)
					}
				} else {
					// Move one grapheme cluster to the left.
					t.moveCursor(t.cursor.row, t.cursor.actualColumn-1)
				}
				newLastAction = taActionBackspace
			} else {
				// Move the cursor back by one word.
				t.moveWordLeft(false)
			}

			// Remove that last grapheme cluster.
			if t.cursor.pos != beforeCursor.pos {
				t.cursor, beforeCursor = beforeCursor, t.cursor                                                 // So we put the right position on the stack.
				t.cursor.pos = t.replace(beforeCursor.pos, t.cursor.pos, "", t.lastAction == taActionBackspace) // Delete the character.
				t.cursor.row = -1
				t.truncateLines(beforeCursor.row - 1)
				t.findCursor(true, beforeCursor.row-1)
			}
			t.selectionStart = t.cursor
		case tcell.KeyDelete, tcell.KeyCtrlD: // Delete forward.
			from, to, row := t.getSelection()
			if from != to {
				// Simply delete the current selection.
				t.cursor.pos = t.replace(from, to, "", false)
				t.cursor.row = -1
				t.truncateLines(row - 1)
				t.findCursor(true, row)
				t.selectionStart = t.cursor
				break
			}

			if t.cursor.pos[0] != 1 {
				_, _, _, _, endPos, _ := t.step("", t.cursor.pos, t.cursor.pos)
				t.cursor.pos = t.replace(t.cursor.pos, endPos, "", t.lastAction == taActionDelete) // Delete the character.
				t.cursor.pos[2] = endPos[2]
				t.truncateLines(t.cursor.row - 1)
				t.findCursor(true, t.cursor.row)
				newLastAction = taActionDelete
			}
			t.selectionStart = t.cursor
		case tcell.KeyCtrlK: // Delete everything under and to the right of the cursor until before the next newline character.
			pos := t.cursor.pos
			endPos := pos
			var cluster, text string
			for pos[0] != 1 {
				var boundaries int
				oldPos := pos
				cluster, text, boundaries, _, pos, endPos = t.step(text, pos, endPos)
				if boundaries&uniseg.MaskLine == uniseg.LineMustBreak {
					if uniseg.HasTrailingLineBreakInString(cluster) {
						pos = oldPos
					}
					break
				}
			}
			t.cursor.pos = t.replace(t.cursor.pos, pos, "", false)
			row := t.cursor.row
			t.cursor.row = -1
			t.truncateLines(row - 1)
			t.findCursor(true, row)
			t.selectionStart = t.cursor
		case tcell.KeyCtrlW: // Delete from the start of the current word to the left of the cursor.
			pos := t.cursor.pos
			t.moveWordLeft(true)
			t.cursor.pos = t.replace(t.cursor.pos, pos, "", false)
			row := t.cursor.row - 1
			t.cursor.row = -1
			t.truncateLines(row)
			t.findCursor(true, row)
			t.selectionStart = t.cursor
		case tcell.KeyCtrlU: // Delete the current line.
			t.deleteLine()
			t.selectionStart = t.cursor
		case tcell.KeyCtrlL: // Select everything.
			t.selectionStart.row, t.selectionStart.column, t.selectionStart.actualColumn = 0, 0, 0
			t.selectionStart.pos = [3]int{t.spans[0].next, 0, -1}
			row := t.cursor.row
			t.cursor.row = -1
			t.cursor.pos = [3]int{1, 0, -1}
			t.findCursor(false, row)
		case tcell.KeyCtrlQ: // Copy to clipboard.
			if t.cursor != t.selectionStart {
				t.copyToClipboard(t.getSelectedText())
				t.selectionStart = t.cursor
			}
		case tcell.KeyCtrlX: // Cut to clipboard.
			if t.cursor != t.selectionStart {
				t.copyToClipboard(t.getSelectedText())
				from, to, row := t.getSelection()
				t.cursor.pos = t.replace(from, to, "", false)
				t.cursor.row = -1
				t.truncateLines(row - 1)
				t.findCursor(true, row)
				t.selectionStart = t.cursor
			}
		case tcell.KeyCtrlV: // Paste from clipboard.
			from, to, row := t.getSelection()
			t.cursor.pos = t.replace(from, to, t.pasteFromClipboard(), false)
			t.cursor.row = -1
			t.truncateLines(row - 1)
			t.findCursor(true, row)
			t.selectionStart = t.cursor
		case tcell.KeyCtrlZ: // Undo.
			if t.nextUndo <= 0 {
				break
			}
			for t.nextUndo > 0 {
				t.nextUndo--
				undo := t.undoStack[t.nextUndo]
				t.spans[undo.originalBefore], t.spans[undo.before] = t.spans[undo.before], t.spans[undo.originalBefore]
				t.spans[undo.originalAfter], t.spans[undo.after] = t.spans[undo.after], t.spans[undo.originalAfter]
				t.cursor.pos, t.undoStack[t.nextUndo].pos = undo.pos, t.cursor.pos
				t.length, t.undoStack[t.nextUndo].length = undo.length, t.length
				if !undo.continuation {
					break
				}
			}
			t.cursor.row = -1
			t.truncateLines(0) // This is why Undo is expensive for large texts. (t.lineStarts can get largely unusable after an undo.)
			t.findCursor(true, 0)
			t.selectionStart = t.cursor
			if t.changed != nil {
				defer t.changed()
			}
		case tcell.KeyCtrlY: // Redo.
			if t.nextUndo >= len(t.undoStack) {
				break
			}
			for t.nextUndo < len(t.undoStack) {
				undo := t.undoStack[t.nextUndo]
				t.spans[undo.originalBefore], t.spans[undo.before] = t.spans[undo.before], t.spans[undo.originalBefore]
				t.spans[undo.originalAfter], t.spans[undo.after] = t.spans[undo.after], t.spans[undo.originalAfter]
				t.cursor.pos, t.undoStack[t.nextUndo].pos = undo.pos, t.cursor.pos
				t.length, t.undoStack[t.nextUndo].length = undo.length, t.length
				t.nextUndo++
				if t.nextUndo < len(t.undoStack) && !t.undoStack[t.nextUndo].continuation {
					break
				}
			}
			t.cursor.row = -1
			t.truncateLines(0) // This is why Redo is expensive for large texts. (t.lineStarts can get largely unusable after an undo.)
			t.findCursor(true, 0)
			t.selectionStart = t.cursor
			if t.changed != nil {
				defer t.changed()
			}
		}
	})
}

// MouseHandler returns the mouse handler for this primitive.
func (t *TextArea) MouseHandler() func(action MouseAction, event *tcell.EventMouse, setFocus func(p Primitive)) (consumed bool, capture Primitive) {
	return t.WrapMouseHandler(func(action MouseAction, event *tcell.EventMouse, setFocus func(p Primitive)) (consumed bool, capture Primitive) {
		if t.disabled {
			return false, nil
		}

		x, y := event.Position()
		rectX, rectY, _, _ := t.GetInnerRect()
		if !t.InRect(x, y) {
			return false, nil
		}

		// Trigger a "moved" event at the end if requested.
		if t.moved != nil {
			selectionStart, cursor := t.selectionStart, t.cursor
			defer func() {
				if selectionStart != t.selectionStart || cursor != t.cursor {
					t.moved()
				}
			}()
		}

		// Turn mouse coordinates into text coordinates.
		labelWidth := t.labelWidth
		if labelWidth == 0 && t.label != "" {
			labelWidth = TaggedStringWidth(t.label)
		}
		column := x - rectX - labelWidth
		row := y - rectY
		if !t.wrap {
			column += t.columnOffset
		}
		row += t.rowOffset

		// Process mouse actions.
		switch action {
		case MouseLeftDown:
			t.moveCursor(row, column)
			if event.Modifiers()&tcell.ModShift == 0 {
				t.selectionStart = t.cursor
			}
			setFocus(t)
			consumed = true
			capture = t
			t.dragging = true
		case MouseMove:
			if !t.dragging {
				break
			}
			t.moveCursor(row, column)
			consumed = true
		case MouseLeftUp:
			t.moveCursor(row, column)
			consumed = true
			capture = nil
			t.dragging = false
		case MouseLeftDoubleClick: // Select word.
			// Left down/up was already triggered so we are at the correct
			// position.
			t.moveWordLeft(false)
			t.selectionStart = t.cursor
			t.moveWordRight(true, false)
			consumed = true
		case MouseScrollUp:
			if t.rowOffset > 0 {
				t.rowOffset--
			}
			consumed = true
		case MouseScrollDown:
			t.rowOffset++
			if t.rowOffset >= len(t.lineStarts) {
				t.rowOffset = len(t.lineStarts) - 1
				if t.rowOffset < 0 {
					t.rowOffset = 0
				}
			}
			consumed = true
		case MouseScrollLeft:
			if t.columnOffset > 0 {
				t.columnOffset--
			}
			consumed = true
		case MouseScrollRight:
			t.columnOffset++
			if t.columnOffset >= t.widestLine {
				t.columnOffset = t.widestLine - 1
				if t.columnOffset < 0 {
					t.columnOffset = 0
				}
			}
			consumed = true
		}

		return
	})
}