Fix regression where tcell renderer not clearing the preview window

src/terminal.go

@@ -1942,9 +1942,14 @@ func (t *Terminal) renderPreviewArea(unchanged bool) {
 		t.pwindow.MoveAndClear(0, 0) // Clear scroll offset display
 	} else {
 		t.previewed.filled = false
-		// We don't erase the window here to avoid flickering during scroll
-		t.pwindow.DrawBorder()
-		t.pwindow.Move(0, 0)
+		// We don't erase the window here to avoid flickering during scroll.
+		// However, tcell renderer uses double-buffering technique and there's no
+		// flickering. So we just erase the window and make the rest of the code
+		// simpler.
+		if !t.pwindow.EraseMaybe() {
+			t.pwindow.DrawBorder()
+			t.pwindow.Move(0, 0)
+		}
 	}
 
 	height := t.pwindow.Height()

src/tui/light.go

@@ -1109,3 +1109,7 @@ func (w *LightWindow) Erase() {
 	w.FinishFill()
 	w.Move(0, 0)
 }
+
+func (w *LightWindow) EraseMaybe() bool {
+	return false
+}

src/tui/tcell.go

@@ -555,6 +555,11 @@ func (w *TcellWindow) Erase() {
 	fill(w.left-1, w.top, w.width+1, w.height-1, w.normal, ' ')
 }
 
+func (w *TcellWindow) EraseMaybe() bool {
+	w.Erase()
+	return true
+}
+
 func (w *TcellWindow) Enclose(y int, x int) bool {
 	return x >= w.left && x < (w.left+w.width) &&
 		y >= w.top && y < (w.top+w.height)

src/tui/tui.go

@@ -526,6 +526,7 @@ type Window interface {
 	Fill(text string) FillReturn
 	CFill(fg Color, bg Color, attr Attr, text string) FillReturn
 	Erase()
+	EraseMaybe() bool
 }
 
 type FullscreenRenderer struct {