When pinch-zooming, scrollbar thumbs are scaled asynchronously to approximate what they will look like when we repaint at the next resolution. Our current scaling logic works on Android, where the scroll thumb length is basically scaled proportionally to the page length. On desktop, however, scroll thumbs appear to have a minimum length, making the scaling logic not exactly proportional. The resulting inaccuracy is not super noticeable, but we do need to resolve it to get [these reftests](https://searchfox.org/mozilla-central/rev/ddb81c7a43ffada1f6cb4200c4f625e50e44dcf3/gfx/layers/apz/test/reftest/reftest.list#13-14) to pass (or else significantly up the fuzz factor on them).
Bug 1554795 Comment 0 Edit History
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When pinch-zooming, scrollbar thumbs are scaled asynchronously to approximate what they will look like when we repaint at the new resolution. Our current scaling logic works on Android, where the scroll thumb length is basically scaled proportionally to the page length. On desktop, however, scroll thumbs appear to have a minimum length, making the scaling logic not exactly proportional. The resulting inaccuracy is not super noticeable, but we do need to resolve it to get [these reftests](https://searchfox.org/mozilla-central/rev/ddb81c7a43ffada1f6cb4200c4f625e50e44dcf3/gfx/layers/apz/test/reftest/reftest.list#13-14) to pass (or else significantly up the fuzz factor on them).