This can be fixed very simply by setting `clipToBounds` on the `NonDraggableView`s we create in `ViewRegion::UpdateRegion`(https://searchfox.org/mozilla-central/rev/630d2c45fc127a44756e3cca8cef51c35654a4b4/widget/cocoa/ViewRegion.mm#20). That's not as satisfying as I would like, because it seems to imply that the problem is some kind of view added to one of the `NonDraggableView` objects, and I don't see where that could be done. I'll post the fix here and we'll discuss in review if we want to push for a better-understood solution.
Bug 1856452 Comment 3 Edit History
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This can be fixed very simply by setting `clipToBounds` on the `NonDraggableView`s we create in [`ViewRegion::UpdateRegion`](https://searchfox.org/mozilla-central/rev/630d2c45fc127a44756e3cca8cef51c35654a4b4/widget/cocoa/ViewRegion.mm#20). That's not as satisfying as I would like, because it seems to imply that the problem is some kind of view added to one of the `NonDraggableView` objects, and I don't see where that could be done. I'll post the fix here and we'll discuss in review if we want to push for a better-understood solution.