Fix TextArea SetContent method

SetContent should overwrite the model, not append to it.

Add a test to check model's width and height after SetContent calls.

views/textarea.go

@@ -90,10 +90,11 @@ func (m *linesModel) GetCursor() (int, int, bool, bool) {
 func (ta *TextArea) SetLines(lines []string) {
 	ta.Init()
 	m := ta.model
+	m.width =0
 
 	// extend slice before using m.runes[row] to avoid panic
 	slice := make([][]rune, len(lines))
-	m.runes = append(m.runes, slice...)
+	m.runes = slice
 
 	for row, line := range lines {
 		for _, ch := range line {

views/textarea_test.go

@@ -0,0 +1,17 @@
+package views
+
+import "testing"
+
+func TestSetContent(t *testing.T) {
+	ta := &TextArea{}
+
+	ta.SetContent("This is a quite long line.")  // This line is longer than 11.
+	ta.SetContent("Four.\nFive...\n...and Six.") //"...and Six." should be 11 long.
+
+	if ta.model.height != 3 {
+		t.Errorf("Incorrect height: %d, expected: %d", ta.model.height, 3)
+	}
+	if ta.model.width != 11 {
+		t.Errorf("Incorrect width: %d, expected: %d", ta.model.width, 11)
+	}
+}