CompareScreenshotTest: mitigate a race condition causing tests to fail

[stephengold]

When I build the project on my Linux desktop, the "hideAndShowCanvas" test usually fails due to a race condition in CompareScreenshotTest.compareWithScreenshot(). Because these tests capture screenshots in a hard loop, it's possible for 2 successive captures to match perfectly before the screen state has actually stabilized.

This PR mitigates the risk of this by adding a half-second delay between screenshots. It's still possible a window manager might animate transitions in such a way that screenshots captured half a second apart match, even when the animation is incomplete. However, empirical testing suggests this change greatly reduces the likelihood of test failure, at the cost of tests taking slightly longer to run.

[gemini-code-assist]

Summary of Changes

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This pull request aims to enhance the reliability of screenshot comparison tests by addressing a race condition where tests could pass prematurely due to rapid, unstable screen state changes. By implementing a short delay, the system is given more time to stabilize, ensuring more accurate and consistent test results, albeit with a minor increase in test execution duration.

Highlights

• Test Stability: Introduced a 500-millisecond delay between screenshot captures in CompareScreenshotTest.compareWithScreenshot() to mitigate a race condition that caused tests to fail, especially on Linux, by allowing the screen state to stabilize.

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[gemini-code-assist]

Code Review

The pull request addresses a race condition in screenshot tests by introducing a delay between captures. This is a good approach to stabilize test results on systems with window animations. However, the implementation of the delay can be made more efficient by using Thread.sleep() instead of a busy-wait loop with Thread.yield().