Created attachment 387647 [details]
Demonstration of the minimal example
In earlier versions of macOS Safari (and current iOS Safari), the fixed header would stay fixed **including** the elastic overscroll. Now, it's not fixed when overscroll is in progress (like Chrome - which seems incorrect).
The problem here is that positioning libraries using `position: fixed` include window scroll to position elements, and Safari reports the overscroll included in the window.pageYOffset value, but since it's no longer fixed with it, the position is wrong when elastic scroll is in progress.
I think we broke this in the big scrolling tree rewrite.
Created attachment 388505 [details]
Created attachment 388508 [details]
Comment on attachment 388508 [details]
Rejecting attachment 388508 [details] from commit-queue.
Failed to run "['/Volumes/Data/EWS/WebKit/Tools/Scripts/webkit-patch', '--status-host=webkit-queues.webkit.org', '--bot-id=webkit-cq-02', 'validate-changelog', '--check-oops', '--non-interactive', 388508, '--port=mac']" exit_code: 1 cwd: /Volumes/Data/EWS/WebKit
ChangeLog entry in LayoutTests/ChangeLog is not at the top of the file.
Full output: https://webkit-queues.webkit.org/results/13310483
This was partially reverted in r256400.