Looks like a clippableElement bug: https://youtu.be/Dh-GKphcrVQ
Description
Description
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This doesn't seem to be related to the overflow-x: auto rule on the popup body (workaround for T130153): if I remove that rule, the collapsing behavior is the same. Perhaps it's related to the clipped element and container not being the same element (they usually are, but they can be different, and this is the only widget that uses that feature).
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This doesn't happen any more. It might have been the same bug as T287132: Notifications dropdown obscured in Timeless, Monobook, new Vector at small resolutions, although you filed it 5 years earlier, so I'm not completely sure.