Make webgl behave better with session history
This prevents the compositor from animating pages that are not actually visible, so pages using webgl do not needlessly impact the performance of the rest of the browser. Additionally, this fixes a problem that was alluded to in [this code](b5228c098b/src/draw_buffer.rs (L282-L285)), causing Servo to delete arbitrary resources when a GC occurred in content that used three.js.
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix#22987 and fix#22977 and fix#20934 and fix#20953 and fix#20930 and fix#20950 and fix#20924
- [x] There are tests for these changes
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Ensure transparent hit test region is sized and positioned correctly.
This addresses some edge cases that were missed by #22156. Specifically, in some cases the transparent region was being placed in the display list in front of the scrollable content, rather than behind it, and the size of the region did not account for parent stacking context.
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix#22073 and fix#22216.
- [x] There are tests for these changes
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Remove forked referrer policy tests
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- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#22663
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Implement HTMLMediaElement poster attribute
- [X] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix#22288
- [x] There are tests for these changes
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Implement DOM APIs for ChannelSplitterNode
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Based on #21591. Fixes#21558. I tried to update the expected results for WPT using "./mach update-wpt"; let me know if I got something wrong.
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- [x] These changes fix the second half of #21558
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- [x] There are web-platform-tests tests for these changes
- [ ] These changes do not require tests because ___
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