"I know of no good reasons why overflow would have to apply to the rest of the frame." https://gerrit.wikimedia.org/r/#/c/282667/, T132306: UI elements of video.js player are hidden by thumbnail frame
The "overflow: hidden" created a block formatting context, which had a visible effect on floating content.
For example see https://de.wikipedia.org/wiki/Unkel#Veranstaltungen. Depending on your screen size, edit that section and remove some of the list items and preview.
Expected result (with overflow: hidden on .thumbinner): The panorama is completely below the right floating image.
Actual result: The floating image overlaps the border of the panorama.