Don't remove the window frame totally, only remove the title bar instead
(including the top border).
We can resize outside our window just like any other normal window in
this way. And many strange problems caused by the removal of window
frame are gone such as background flicker.
However, we will need to paint the top frame border ourself. Otherwise
the window doesn't look very good.
Signed-off-by: Yuhang Zhao <2546789017@qq.com>