RFR: 8187759: Background not refreshed when painting over a transparent JFrame [v4]

[Sergey Bylokhov]

On Tue, 20 Feb 2024 05:36:55 GMT, Tejesh R <tr at openjdk.org> wrote:

>> Since this is an intermediate buffer its content should not affect the rendering, so it should contain only the image rendered by the component itself.
>
> This how it is done in  [RepaintManager](https://github.com/openjdk/jdk/blob/69a11c7f7ea7c4195a8ee56391bdf04c75bd8156/src/java.desktop/share/classes/javax/swing/RepaintManager.java#L1711), it uses background color of the component.

That code was added by me to implement the shaped windows on macOS, that is the only platform we support the translucent backbuffer in RepaintManager. For the translucent backbuffer and even opaque component it is necessary to clear its content since the rendering iside of the components assumes the simple "fillrect" will clear the component which is not true if the composite is not Src.
But probably at that place we always start rendering from the Window? in that case the usage of background is a right thing. I am not sure that the usage of the component background is a correct thing, and probably 0,0,0 should work better. 

I suggest to check that we do not fill the background twice, by the code added in the patch, and the code inside of the paintToOffscreen(). For example if there are the container and the component inside, if the component is not opaque and has no any content, will the container be fully visible? or we will see the background of the component(which should not be there).

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PR Review Comment: https://git.openjdk.org/jdk/pull/17081#discussion_r1495342708