Popups currently, as of v0.31.3, rely on box-shadow, which is quite well-supported, but implies a UI glitch, as the shadow is rendered only *drumroll* on boxes.
The callouts are not covered by it.
Proposal
Use [[ https://caniuse.com/#feat=css-filters | filter: drop-shadow ]] where applicable in progressive enhancement manner.
v0.31.3 | Proposed |
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WikimediaUI theme | |
Apex theme | |