Improve preview window rendering

- Fix incorrect display of the last line when more than a line is
  wrapped above
- Avoid unnecessary flickering of the window

src/tui/tcell.go

@@ -164,6 +164,10 @@ func (w *TcellWindow) X() int {
 	return w.lastX
 }
 
+func (w *TcellWindow) Y() int {
+	return w.lastY
+}
+
 func (r *FullscreenRenderer) DoesAutoWrap() bool {
 	return false
 }