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/tui.go

@@ -236,6 +236,7 @@ type Window interface {
 	Close()
 
 	X() int
+	Y() int
 	Enclose(y int, x int) bool
 
 	Move(y int, x int)