Thanks; it looks like that puts us more in-line with Chrome's performance on these testcases. (Still a little slower, but not massively so.) I looked at the "50k: https://share.firefox.dev/4hpYbbn (5s)" profile; one thing I'm noticing as potentially-optimizable there is `mozilla::DepthOrderedFrameList::Add` (as part of `mozilla::PresShell::FrameNeedsReflow`) -- that spends nearly all of its time in `mozilla::DepthOrderedFrameList::Contains`, because we've got a massive array there (containing all of the reflow roots, and there's one for each scrollbar). We could maybe optimize that by switching to use a `set`-like structure for `contains()`-checking at some point, but I'm also not sure this is common enough behavior to worry about doing that.... So: I don't think it's worth worrying too much about improvements here, but one last thing that'd be worth investigating is how Chromium (whether Chrome or Edge) fares when configured to use overlay scrollbars, for an apples-to-apples comparison with the Firefox results in comment 0. I think there's some sort of setting or flag to turn that on in Chrome and/or Edge (maybe just Edge?), if you're interested and able to find that.
Bug 1943862 Comment 5 Edit History
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Thanks; it looks like that puts us more in-line with Chrome's performance on these testcases. (Still a little slower, but not massively so.) I looked at Comment 4's "50k: https://share.firefox.dev/4hpYbbn (5s)" profile; one thing I'm noticing as potentially-optimizable there is `mozilla::DepthOrderedFrameList::Add` (as part of `mozilla::PresShell::FrameNeedsReflow`) -- that spends nearly all of its time in `mozilla::DepthOrderedFrameList::Contains`, because we've got a massive array there (containing all of the reflow roots, and there's one for each scrollbar). We could maybe optimize that by switching to use a `set`-like structure for `contains()`-checking at some point, but I'm also not sure this is common enough behavior to worry about doing that.... So: I don't think it's worth worrying too much about improvements here, but one last thing that'd be worth investigating is how Chromium (whether Chrome or Edge) fares when configured to use overlay scrollbars, for an apples-to-apples comparison with the Firefox results in comment 0. I think there's some sort of setting or flag to turn that on in Chrome and/or Edge (maybe just Edge?), if you're interested and able to find that.