Primitive widths/heights should not become negative. Also fixed a

TextView issue where purging led to panics. Fixes #306

box.go

@@ -88,7 +88,8 @@ func (b *Box) GetRect() (int, int, int, int) {
 }
 
 // GetInnerRect returns the position of the inner rectangle (x, y, width,
-// height), without the border and without any padding.
+// height), without the border and without any padding. Width and height values
+// will clamp to 0 and thus never be negative.
 func (b *Box) GetInnerRect() (int, int, int, int) {
 	if b.innerX >= 0 {
 		return b.innerX, b.innerY, b.innerWidth, b.innerHeight
@@ -100,10 +101,17 @@ func (b *Box) GetInnerRect() (int, int, int, int) {
 		width -= 2
 		height -= 2
 	}
-	return x + b.paddingLeft,
-		y + b.paddingTop,
-		width - b.paddingLeft - b.paddingRight,
-		height - b.paddingTop - b.paddingBottom
+	x, y, width, height = x+b.paddingLeft,
+		y+b.paddingTop,
+		width-b.paddingLeft-b.paddingRight,
+		height-b.paddingTop-b.paddingBottom
+	if width < 0 {
+		width = 0
+	}
+	if height < 0 {
+		height = 0
+	}
+	return x, y, width, height
 }
 
 // SetRect sets a new position of the primitive. Note that this has no effect
@@ -318,6 +326,12 @@ func (b *Box) Draw(screen tcell.Screen) {
 		b.innerHeight += b.innerY
 		b.innerY = 0
 	}
+	if b.innerWidth < 0 {
+		b.innerWidth = 0
+	}
+	if b.innerHeight < 0 {
+		b.innerHeight = 0
+	}
 }
 
 // Focus is called when this primitive receives focus.

textview.go

@@ -939,8 +939,13 @@ func (t *TextView) Draw(screen tcell.Screen) {
 	// If this view is not scrollable, we'll purge the buffer of lines that have
 	// scrolled out of view.
 	if !t.scrollable && t.lineOffset > 0 {
-		t.buffer = t.buffer[t.index[t.lineOffset].Line:]
+		if t.lineOffset <= len(t.index) {
+			t.buffer = nil
+		} else {
+			t.buffer = t.buffer[t.index[t.lineOffset].Line:]
+		}
 		t.index = nil
+		t.lineOffset = 0
 	}
 }