Update: this still happens in Zotero 11.0-beta.2 (Windows 11), in the library's item pane too.

I found the cause. After a click on a sidenav button, or with a pinned section, the pane gets bottom padding (--min-scroll-height) so the section can sit at the top. The scroll handler in itemDetails.js (_handleContainerScroll) then shrinks that padding to scrollTop + clientHeight. clientHeight is a whole number, but the scrolling area can start on a half pixel and be x.5 px tall. Here that happens with the Bibliography Entry header, whose lines are 19.5 px high, on an item whose entry takes an odd number of lines. The padding then ends 0.5 px below the real bottom, scrollHeight rounds up by 1 px, and every further scroll event at the bottom repeats it: +1 px per event, indefinitely.

Steps:
1. Set the item pane header to Bibliography Entry.
2. Select an item whose entry takes an odd number of lines (1 or 3).
3. Click a sidenav button, or pin a section.
4. Scroll to the bottom of the item pane and keep scrolling.

Using the box's real height (this._paneParent.getBoundingClientRect().height) instead of clientHeight in that handler stops it. I tested this by writing the corrected value right after the handler runs.