Bugzilla

STR:
1. Load https://bug1912854.bmoattachments.org/attachment.cgi?id=9419989
2. Scroll down.

EXPECTED RESULTS:
As the dynamic toolbar scrolls out of view, the header should remain snapped to the top of the screen.

ACTUAL RESULTS:
As the dynamic toolbar scrolls out of view, the header shifts downwards towards the center of the page. Then when the dynamic toolbar entirely disappears, the header snaps back to the top of the screen.

NOTES
I'm using "testcase 1" from bug 1912854, but this bug goes back further than that bug -- i.e. it affects official Firefox release 129.0.2 (and probably further back too).  There are a range of nightlies that experience a worse bug (bug 1912854 specifically) where the header bar *remains* detached - that's fixed as of today, though, so we're back to a state where the header simply detaches temporarily.

It looks like maybe what's happening is: as we transition to having a larger viewport, we vertically-center the "old" fixed-position-containing-block within whatever our current intermediate screen size is; and then we do a clean switch when we've arrived at the final viewport size.  Chrome does something like this too, but they don't vertically-center -- they seem to just snap the "old" fixed-position containing block to the top of the screen as their dynamic toolbar is animating away.

STR:
1. Load https://bug1912854.bmoattachments.org/attachment.cgi?id=9419989
2. Scroll down.

EXPECTED RESULTS:
As the dynamic toolbar scrolls out of view, the header should remain snapped to the top of the screen.

ACTUAL RESULTS:
As the dynamic toolbar scrolls out of view, the header shifts downwards towards the center of the page. Then when the dynamic toolbar entirely disappears, the header snaps back to the top of the screen.

NOTES
The linked testcase is "testcase 1" from bug 1912854, but this bug goes back further than that bug -- i.e. it affects official Firefox release 129.0.2 (and probably further back too).  There are a range of nightlies that experience a worse bug (bug 1912854 specifically) where the header bar *remains* detached - that's fixed as of today, though, so we're back to a state where the header simply detaches temporarily.

It looks like maybe what's happening is: as we transition to having a larger viewport, we vertically-center the "old" fixed-position-containing-block within whatever our current intermediate screen size is; and then we do a clean switch when we've arrived at the final viewport size.  Chrome does something like this too, but they don't vertically-center -- they seem to just snap the "old" fixed-position containing block to the top of the screen as their dynamic toolbar is animating away.