RFR: JDK-8303950: [macos]Translucent Windows Flicker on Repaint [v2]

[Andrey Turbanov]

On Wed, 7 Jun 2023 19:19:59 GMT, Jeremy <duke at openjdk.org> wrote:

>> # Problem Summary
>> 
>> For non-opaque windows, Window#paint calls `gg.fillRect(0, 0, getWidth(), getHeight())` before `super.paint(g)`.
>> 
>> This can cause flickering on Mac, and the flickering seems to have gotten much worse in recent JVMs. (See movie attachments to original ticket.)
>> 
>> # Discussion
>> 
>> This is my 2nd PR for this ticket. The original is [here](https://github.com/openjdk/jdk/pull/12993); that proposed change was IMO more invasive/scarier. It was auto-closed after 8 weeks of inactivity, and I'd rather offer this PR instead.
>> 
>> In that previous discussion Alan Snyder framed the core problem as a "lack of synchronization" (see [comment here](https://github.com/openjdk/jdk/pull/12993#issuecomment-1467528061)). If the monitor refreshes/flushes after we've called `fillRect` *but before we finish `super.paint`*: it makes sense that we'd see a flicker.
>> 
>> I agree with Alan, but I think the problem can also be framed as a mixing-Swing-with-AWT issue. (Which IMO will involve an easier fix.)
>> 
>> This PR is a low-risk change (relative to the previous PR) that intercepts calls to repaint a Window that is also RootPaneContainer. Now we'll redirect those calls to paint the JRootPane instead. This means we'll exclusively paint within Swing's/RepaintManager's double-buffered architecture, so we bypass the risky call to `fillRect` on the screen's Graphics2D. (And this change occurs within RepaintManager, so we're clearly already in Swing's architecture.)
>> 
>> So with this change: we paint everything to the double-buffer, and the *only time* we paint to the Window's Graphics2D is when have set up a AlphaComposite.Src and replace its contents with our buffer's contents.
>> 
>> # Tests
>> 
>> This PR includes a new test for 8303950 itself. This is pretty self-explanatory: we repaint a trivial animation for a few seconds and use the Robot to see if a pixel is the expected color.
>> 
>> This PR also includes a test called `bug8303950_legacyWindowPaintBehavior` that creates a grid of 4 windows with varying opacity/backgrounds:
>> 
>> <img width="805" alt="image" src="https://github.com/openjdk/jdk/assets/7669569/497c21a0-ed18-413f-a5c7-3ea146644fd6">
>> 
>> I was surprised by how these windows rendered, but I don't think that's worth debating here. This test simply makes sure that we preserve this preexisting behavior. The broad "rules" appear to be:
>> 1. If a JComponent identifies as opaque (see `JComponent.isOpaque`) then the JComponent's background is used. (I...
>
> Jeremy has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - 8303950: Adding "@key headful"
>    
>    Based on prrace's feedback https://github.com/openjdk/jdk/pull/14363#issuecomment-1581365662
>  - 8303950: remove call to Window#toBack()
>    
>    toBack() moves the Window behind *everything*. If it moved windows only behind other windows in the Swing app that would be great, but that's not what it does.

test/jdk/javax/swing/RepaintManager/8303950/bug8303950_legacyWindowPaintBehavior.java line 68:

> 66:                 Window w2 = createWindow( WINDOW_BACKGROUND, ROOTPANE_BACKGROUND, x + 400, y, 400, 400, false, "window 2");
> 67:                 Window w3 = createWindow( WINDOW_BACKGROUND, null, x, y + 400, 400, 400, true, "window 3");
> 68:                 Window w4 =  createWindow( WINDOW_BACKGROUND, ROOTPANE_BACKGROUND, x + 400, y + 400, 400, 400, true, "window 4");

Suggestion:

                Window w4 = createWindow( WINDOW_BACKGROUND, ROOTPANE_BACKGROUND, x + 400, y + 400, 400, 400, true, "window 4");

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PR Review Comment: https://git.openjdk.org/jdk/pull/14363#discussion_r1232733209