We already rely on it applying only to XULPopupElement's. Make it
explicit and remove the CSS value.

Backed out for causing mochitest failures on win7

• Backout link
• Push with failures
• Failure Log
• Failure line: TEST-UNEXPECTED-FAIL | browser/base/content/test/static/browser_sentence_case_strings.js | Test timed out -

Backed out for causing failures on browser_history_after_appMenu.js

• backout: https://hg.mozilla.org/integration/autoland/rev/11d7f4ccadeda544305c611cfa9d959192d7f5b5
• push: https://treeherder.mozilla.org/jobs?repo=autoland&group_state=expanded&selectedTaskRun=bT5ojuRETQes3bVp1QZDuA.0&revision=713e6f0fbd20db80bad6413d005f8e82d8d37898
• failure log: https://treeherder.mozilla.org/logviewer?job_id=396076080&repo=autoland&lineNumber=1672

[task 2022-11-09T18:01:42.041Z] 18:01:42     INFO - TEST-START | browser/components/customizableui/test/browser_history_after_appMenu.js
[task 2022-11-09T18:02:27.053Z] 18:02:27     INFO - TEST-INFO | started process screenshot
[task 2022-11-09T18:02:27.110Z] 18:02:27     INFO - TEST-INFO | screenshot: exit 0
[task 2022-11-09T18:02:27.111Z] 18:02:27     INFO - Buffered messages logged at 18:01:42
[task 2022-11-09T18:02:27.111Z] 18:02:27     INFO - Entering test bound test_history_after_appMenu
[task 2022-11-09T18:02:27.112Z] 18:02:27     INFO - Checking if element has non-0 size
[task 2022-11-09T18:02:27.112Z] 18:02:27     INFO - TEST-PASS | browser/components/customizableui/test/browser_history_after_appMenu.js | The panel is closed to begin with. - "closed" == "closed" - 
[task 2022-11-09T18:02:27.113Z] 18:02:27     INFO - Buffered messages finished
[task 2022-11-09T18:02:27.114Z] 18:02:27     INFO - TEST-UNEXPECTED-FAIL | browser/components/customizableui/test/browser_history_after_appMenu.js | Test timed out - 
[task 2022-11-09T18:02:27.114Z] 18:02:27     INFO - Not taking screenshot here: see the one that was previously logged
[task 2022-11-09T18:02:27.115Z] 18:02:27     INFO - TEST-UNEXPECTED-FAIL | browser/components/customizableui/test/browser_history_after_appMenu.js | Uncaught exception received from previously timed out test - ViewShown listener on #appMenu-protonMainView not removed before the end of test
[task 2022-11-09T18:02:27.381Z] 18:02:27     INFO - GECKO(4816) | MEMORY STAT | vsize 1481MB | vsizeMaxContiguous 131MB | residentFast 447MB | heapAllocated 506MB
[task 2022-11-09T18:02:27.382Z] 18:02:27     INFO - TEST-OK | browser/components/customizableui/test/browser_history_after_appMenu.js | took 45337ms

Bug 1799925 fixes that (via the NS_FRAME_IS_DIRTY fix, presumably)

https://hg.mozilla.org/mozilla-central/rev/d822ff58de6b