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

[Semyon Sadetsky]

On 10/27/2016 2:57 PM, Sergey Bylokhov wrote:
> On 27.10.16 10:51, Semyon Sadetsky wrote:
>>> 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()).
>
> When the backbuffer is disabled: The code in the Window#paint() should 
> be executed when the Window(top level is painted). After that the 
> child components should be painted on top of that(srcOver will be 
> used). Note that we should not fill background of the parent when we 
> paint a child.
It is always painted in case of non-opaque window in 
java.awt.Window#paint(). Are you proposing to change that?
> When the backbuffer is enabled for one components or for all 
> components, we should draw hierarchy of components one by one to the 
> backbuffer(in one step), and then src-blit the backbuffer to the 
> window. Or we can paint only some of components via buffer, but in 
> this case we should clear the buffer to (0,0,0,0), draw the component, 
> then srcover-blit to the parent. It is unclear why we should clear 
> buffer to the parent's color when we draw a child, because the parent 
> itself should draw this background to the backbuffer, or to the window.
Because the  back buffer is used for swing components (JRootPane is back 
buffered) while HW container paint is not back buffered but it should 
contribute to the painting to get the same result as without back-buffering.
Please be more specific on your suggestions.
>
>>>   Should the previous composite be restored after the rect filling?
>> SRC should be the default composite type.
>
> default composite type should be srcOver, and it should be restored 
> before call paintToOffscreen().
>