If the "View history" tab changes size such that becomes too large where it before was not, an endless animation loop of collapse-expand-collapse-etc can be triggered. A resize could be caused by an ordinary zoom (frequently resizing the pixel width of the element by a few pixels), by a change in browser text size preferences (likely to significantly change the width), or by a user script modifying the text content of the element.
This appears to be caused by expandCondition in Vector.js using a cached version of the element's width (collapsibleTabs.data's expandedWidth passed as the eleWidth argument), while collapseCondition is checking .width() each time.
This happens when you enable `Editing mode: show both tabs is enabled in preferences` in VisualEditor and visit https://www.mediawiki.org/wiki/User:Martin_Urbanec?uselang=fr at certain screen resolutions. Demonstrated in image below:
{F5061717}
== Update 2020-05-30
This bug resurfaced as a more prominent regression after the #mw1.35.0-wmf.34 deployment.
Related regression: {T253912}
User video:
https://phab.wmfusercontent.org/file/data/inpa275colq2onvgvluh/PHID-FILE-jsdpug3iah5m6f76nvwx/funnybug.gif (from T253819)
https://www.reddit.com/r/softwaregore/comments/gtbdwu/cant_fit_all_the_buttons_in_the_frame_wikipedia/
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**Version**: unspecified
**Severity**: normal