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

[Semyon Sadetsky]

On 10/27/2016 7:38 PM, Sergey Bylokhov wrote:
> On 27.10.16 19:21, Semyon Sadetsky wrote:
>>>> 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?
>
> It should not be changed. It is painted by the Window at paint() method, 
> which is located in the window class, which paints its own backgound.
>
>>> 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.
>
> As you mention above background of the window will be always cleared 
> by the window itself, and we can get double alpha in the background 
> when we blit the backbuffer to the window. What composite is used when 
> we draw backbuffer to the window?
It is src-over.
>
>>>
>>>>>   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().
>
>