I can pretty easily reproduce this here on Bugzilla, using the "Request information from [other]" select field at the bottom. When the drop-down is opened but needs to be repositioned because it hits the screen edge, it flickers on opening (this also seem to affect all other popups). Once hovered, its background, with exception of the hovered item, shrinks to the minimum size.
Bug 1679979 Comment 2 Edit History
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I can pretty easily reproduce this here on Bugzilla, using the "Request information from [other]" select field at the bottom. When the drop-down is opened but needs to be repositioned because it hits the screen edge, it flickers on opening (this also seem to affect all other popups). Once hovered, its background, with exception of the hovered item, shrinks to the minimum size. Firefox Nightly, GNOME 40.1, Wayland.