[I'm spinning this off from bug 1618029 comment 3. Thanks to @toulousevictor for the bug report over there.]

STR:

1. Click the pin/thumbtack icon at the top left.
2. Load attached testcase (original URL: https://jsfiddle.net/vctls/mac2Ls0d/16/ )
3. Scroll down and hover the green folder icon at the very bottom.

EXPECTED RESULTS:
A "shrink" animation should play on the folder.

ACTUAL RESULTS:

• All of the content from the top of the page seems to snap into the current position of the scrollport
• The folder icon gets thrown off the bottom (creating additional scrollable area)
• If you try to scroll up, there's a bunch of blank space before the content now.

Chrome gives EXPECTED RESULTS.
Firefox Nightly 104 gives ACTUAL RESULTS.

(Note: It seems possible this is a version of https://bugzilla.mozilla.org/show_bug.cgi?id=1748891 )

Here's a reduced testcase.

STR here:

1. Scroll the scrollable area to the bottom.
2. Repeatedly hover/unhover the "Hover me" text.

ACTUAL RESULTS: The scrollbar shows that the scrollable area keeps getting taller (with the content all being moved to the very end).

(In reply to Daniel Holbert [:dholbert] from comment #0)

(Note: It seems possible this is a version of https://bugzilla.mozilla.org/show_bug.cgi?id=1748891 )

The interesting wrinkle here:

• the sticky-positioned thing is also a float
• the content that's "propping up" the furthest scrollable area is not itself sticky-positioned, though it is cleared to be below the sticky float, and that clearance seems to be necessary to trigger the bug. (In the original testcase it's cleared via width:100% which forces it below the float so that it fits; in testcase 2, I clear it explicitly via clear:both.)