It looks like reddit.com/r/rust doesn't get APZ (or at least we appear to becontinuously scene building). Another thing that makes scrolling smooth again is to scroll while another widget (for example the hamberger menu) is open on top of the browser's window. I have a feeling that this might be related to how hit-testing sometimes blocks the main thread, maybe interacting with some transactions with scene building that get in front of the hit testing query. Not sure why we wouldn't be doing these sync hit test queries and or flushes when the profiler is active or when another widget is on top, though.
Bug 1578199 Comment 1 Edit History
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It looks like reddit.com/r/rust doesn't get APZ (or at least we appear to becontinuously scene building). Another thing that makes scrolling smooth again is to scroll while another widget (for example the hamberger menu) is open on top of the browser's window. I have a feeling that this might be related to how hit-testing sometimes blocks the main thread, maybe interacting with some transactions with scene building that get in front of the hit testing query. Not sure why we wouldn't be doing these sync hit test queries and or flushes when the profiler is active or when another widget is on top, though. Edit: The APZ thing is unclear. Definitely worse towards the top of the page where there is some sticky behavior with side menus. Further down the page I can definitely see some frames of scrolling that didn't come with a scene build.