One after you clicked "change" button in the contents, you can see checkerboarding there. A key factor to reproduce this issue is making a sub scroll container be non-scrollable by expanding the container size. Even after becoming non-scrollable, the corresponding APZC keeps alive in APZCTree, and I guess it somehow mis-behaves?
Bug 1781007 Comment 15 Edit History
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One after you clicked "change" button in the contents, you can see checkerboarding there. A key factor to reproduce this issue is making a sub scroll container be non-scrollable by expanding the container size. Even after becoming non-scrollable, the corresponding APZC keeps alive in APZCTree, and I guess it somehow mis-behaves? EDIT: Looks like you shouldn't scroll the container before clicking the button to reproduce the issue.
Once after you clicked "change" button in the contents, you can see checkerboarding there. A key factor to reproduce this issue is making a sub scroll container be non-scrollable by expanding the container size. Even after becoming non-scrollable, the corresponding APZC keeps alive in APZCTree, and I guess it somehow mis-behaves? EDIT: Looks like you shouldn't scroll the container before clicking the button to reproduce the issue.