<Swing Dev> [9] Review request for 8027639: JComboBox's popup leaves tracks after closing

[Semyon Sadetsky]

On 10/25/2016 7:58 PM, Sergey Bylokhov wrote:

> On 25.10.16 14:42, Alexandr Scherbatiy wrote:
>>> When the Swing repaint manager paints Swing to a separate off-screen
>>> buffer the root window is not painted. So in case of non-opaque root
>>> window having transparent background the repainted buffer area
>>> transparency is not restored and previously visible Swing components
>>> can remain visible.
>>>
>>> The solution is to paint the root window's background to the repainted
>>> area in case the root window is non-opaque.
>>   Should the previous composite be restored after the rect filling?
>
> Did you check that if the rootpane background contain non-zero alpha 
> it does not draws twice? The first one when the the root will be draw 
> itself and the second time in the new code when the component will be 
> drawn? probably backbuffer should be initialized to Color(0,0,0,0) 
> instead?
Probably you've confused that root is not an instance of the JRootPane. 
It is a HW container that is parent of the root pane.
But there is an another color discrepancy here. Since the HW container 
is not opaque the background color should not be painted. But we fill 
background to clear the updated area. Probably the fill color should be 
Color(0,0,0,0) in this case but currently the getBackground() color is 
used if painting is executed without back buffer (see 
java.awt.Window#paint()). So, if I change this for the back buffer clear 
the result of painting will be different on different platforms and 
pipes.  Also it seems this is the only way to have transparent but 
colored window background.