Steps to reproduce:
1. Go to https://build.webkit.org/console
2. Scroll down a bit
3. Click on any bubble
4. The popup is not shown, because it's positioned at the top of the page, scroll up and you will see the it there.
showBuildBox() should probably use document.scrollingElement instead of document.body to get the scroll offset.
Shouldn't this be backwards compatible? There might be more websites broken.
(In reply to Carlos Garcia Campos from comment #2)
> Shouldn't this be backwards compatible? There might be more websites broken.
Yes this change was expected to break existing content. See https://lists.webkit.org/pipermail/webkit-dev/2018-January/029857.html
Does the bug reproduce in Chrome?
(In reply to Simon Fraser (smfr) from comment #4)
> Does the bug reproduce in Chrome?
Yes it reproduces in Chrome but for some reason not in Firefox. Maybe they have different interpretation of whether to use quirks mode with XHTML pages.