User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Firefox/91.0

Steps to reproduce:

0. Restart Firefox, possibly with cookies cleared, etc., or in safe-mode.
1. Create a folder with 30+ bookmarks all pointing to (possibly different) URLs containing #LABEL at the end, which should result in Firefox loading the page scrolled to the given <a name> position.
   Sample URL: https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled
2. Open all bookmarks from the folder in new tabs.

Actual results:

Some of those tabs may randomly be rendered empty or (rarely) partially empty (missing lower horizontal part).
Only page's background color is always visible (for the URL above it is white anyway).

To see the page rendered properly you need to refresh it or scroll it.

When scrolled the page is rendered positioned in a wrong place, often not even near where the #LABEL pointed, sometimes at the end - but the vertical scrollbar is always in the right place.

Expected results:

Websites should always be scrolled and rendered properly.

The Bugbug bot thinks this bug should belong to the 'Core::Widget: Gtk' component, and is moving the bug to that component. Please revert this change in case you think the bot is wrong.

To see the page rendered properly you need to refresh it or scroll it.
To see the page rendered properly you need to: focus the address bar (F6) and confirm it again (Enter), or scroll it.

When refreshing the page with F5 it is rendered properly, but in an entirely different place, not where the #LABEL points to.

Thanks for the report!
Confirmed on Gnome Xwayland, Debian Testing, Intel.
Only the first tab is rendered. All other tabs are empty by default, have a scrollbar and show the content when scrolled.
mozregression --launch 91 --pref gfx.webrender.all:true -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled

Seems to be fixed in https://nightly.mozilla.org.
fix range:
bad: all tabs except the first are empty until scrolled
good: all tabs are visible and scrolled to the correct position
$ pip3 install --upgrade mozregression
$ mozregression --find-fix --bad 91 --good 2021-09-27 --pref gfx.webrender.all:true -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled

11:51.27 INFO: First good revision: d400d0a16ce6c20bfe5addb995f610f4881b31e6
11:51.27 INFO: Last bad revision: ab80197101b9c7163739f04019ba64fd08770e4b
11:51.27 INFO: Pushlog:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=ab80197101b9c7163739f04019ba64fd08770e4b&tochange=d400d0a16ce6c20bfe5addb995f610f4881b31e6

d400d0a16ce6c20bfe5addb995f610f4881b31e6 Emilio Cobos Álvarez — Bug 1722487 - Avoid some work for font list updates. r=jfkthame

first regression:
good: good
bad: many tabs are scrolled to the bottom and not to the anchor
MOZ_DISABLE_CONTENT_SANDBOX=1 mozregression --good 78 --bad 2021-01-31 --pref gfx.webrender.all:true -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled

8:59.92 INFO: Last good revision: 69ae5f925fbc67d86a6fda1ed0a4b02ae8fc4588
8:59.92 INFO: First bad revision: 02d875d4f7d769cd03be3175490377dc600355fe
8:59.92 INFO: Pushlog:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=69ae5f925fbc67d86a6fda1ed0a4b02ae8fc4588&tochange=02d875d4f7d769cd03be3175490377dc600355fe

02d875d4f7d769cd03be3175490377dc600355fe Jonathan Kew — Bug 1676966 - Don't block layout on global font fallback; load character maps asynchronously, and then reflow when available. r=lsalzman

second regression:
good: many tabs are scrolled to the bottom and not to the anchor
bad: all tabs except the first are empty until scrolled
MOZ_DISABLE_CONTENT_SANDBOX=1 mozregression --good 78 --bad 91 --pref gfx.webrender.all:true -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled -a https://developer.mozilla.org/en-US/docs/Web/Accessibility/Understanding_WCAG/Text_labels_and_names#headings_must_be_labeled

11:45.80 INFO: Last good revision: a1efddeb1c4bab7227af51be899ef25fcee1a495
11:45.80 INFO: First bad revision: 3bf825df68abe2ab8cca86e7e318d512a4731810
11:45.80 INFO: Pushlog:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=a1efddeb1c4bab7227af51be899ef25fcee1a495&tochange=3bf825df68abe2ab8cca86e7e318d512a4731810

3bf825df68abe2ab8cca86e7e318d512a4731810 Timothy Nikkel — Bug 1687926. Don't request a repaint and set a display port if it's the first time for an azpc getting metrics always via the visualScrollOffsetUpdated path. r=botond