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

[Jeremy Wood]

Alan,

No, I’m not currently set up to test OpenJDK PR’s on other platforms.

I’m fuzzy on exactly what happens automatically in an OpenJDK PR. Are my 
new unit tests automatically run on a variety of platforms? (That is: 
are my new unit tests part of what executes here 
<https://github.com/mickleness/jdk/actions/runs/5197802507/jobs/9386648003>? 
Or does that skip several UI-based unit tests?)

I do have a Windows machine, but IIRC setting up OpenJDK to build on my 
Mac was difficult, and I don’t want to go through that again on Windows 
unless I really have to.

  - Jeremy

------ Original Message ------
>From "Alan Snyder" <javalists at cbfiddle.com>
To "Jeremy" <duke at openjdk.org>
Cc client-libs-dev at openjdk.org
Date 6/7/23, 2:06:23 PM
Subject Re: RFR: JDK-8303950: [macos]Translucent Windows Flicker on 
Repaint

>Are you able to test on platforms other than macOS?  (I’m not, so I can’t help with that.)
>
>   Alan
>
>
>
>>  On Jun 7, 2023, at 10:53 AM, 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. (In this case: that's the opaque green top two windows.) This is probably coming from `ComponentUI.update`
>>  2. If a JRootPane has a translucent background, that color "wins". It doesn't composite on top of a window's translucent background: it simply *replaces* it. (See the blue-ish bottom-right window. Note it's blue -- not purple.) This is probably because the RepaintManager uses `AlphaComposite.Src` in a couple of places to replace its destination Graphics.
>>  3. If a JRootPane has a null background color, then the Window's background color paints as expected. (See the red-ish bottom-left window.)
>>
>>  -------------
>>
>>  Commit messages:
>>  - 8303950: working on occasional unit failures
>>  - 8303950: avoid System.exit(1)
>>  - 8303950: adding opaque white window under test windows
>>  - 8303950: this resolves bug8303950 by adding special RootPaneContainer behavior
>>  - 8303950: adding unit test for legacy window behavior
>>  - 8303950: fixing false negative
>>  - 8303950: adding failing test to identify problem
>>  - Merge pull request #4 from openjdk/master
>>  - Merge pull request #3 from openjdk/master
>>  - Merge pull request #2 from openjdk/master
>>  - ... and 1 more: https://git.openjdk.org/jdk/compare/98a7a60f...37834391
>>
>>  Changes: https://git.openjdk.org/jdk/pull/14363/files
>>  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14363&range=00
>>   Issue: https://bugs.openjdk.org/browse/JDK-8303950
>>   Stats: 409 lines in 3 files changed: 407 ins; 1 del; 1 mod
>>   Patch: https://git.openjdk.org/jdk/pull/14363.diff
>>   Fetch: git fetch https://git.openjdk.org/jdk.git pull/14363/head:pull/14363
>>
>>  PR: https://git.openjdk.org/jdk/pull/14363
>>
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