@dchan and I did some experimentation. We found a few things:

Change #1041171 merged by jenkins-bot:

[mediawiki/extensions/CollaborationKit@master] styles: Remove outdated Firefox proprietary CSS extensions

https://gerrit.wikimedia.org/r/1041171

This bug is still present. The screen shot is of https://en.wikipedia.org/w/index.php?title=User:Jonesey95/sandbox&oldid=1229635601 using Brave for Mac, Vector 2022, logged out, desktop view.

Filed remaining work as T367739

Change #1041782 merged by jenkins-bot:

[mediawiki/core@master] Update OOUI to v0.50.0

https://gerrit.wikimedia.org/r/1041782

Change #1041782 had a related patch set uploaded (by Catrope; author: Catrope):

[mediawiki/core@master] Update OOUI to v0.50.0

https://gerrit.wikimedia.org/r/1041782

Change #1041167 merged by jenkins-bot:

[mediawiki/extensions/Kartographer@master] styles: Remove outdated Firefox proprietary CSS extensions

https://gerrit.wikimedia.org/r/1041167

Change #1041171 had a related patch set uploaded (by VolkerE; author: VolkerE):

[mediawiki/extensions/CollaborationKit@master] styles: Remove outdated Firefox proprietary CSS extensions

https://gerrit.wikimedia.org/r/1041171

Change #1041167 had a related patch set uploaded (by VolkerE; author: VolkerE):

[mediawiki/extensions/Kartographer@master] styles: Remove outdated Firefox proprietary CSS extensions

https://gerrit.wikimedia.org/r/1041167

Change #1038825 had a related patch set uploaded (by VolkerE; author: VolkerE):

[mediawiki/extensions/Forms@master] styles: Remove outdated browsers like MS IE10 Flexbox workarounds

https://gerrit.wikimedia.org/r/1038825

Change #1037873 merged by jenkins-bot:

[mediawiki/core@master] styles: Remove IE11 workarounds and comment hints

https://gerrit.wikimedia.org/r/1037873

Change #1037873 had a related patch set uploaded (by VolkerE; author: VolkerE):

[mediawiki/core@master] styles: Remove IE11 workarounds and comment hints

https://gerrit.wikimedia.org/r/1037873

Change #1037742 merged by jenkins-bot:

[mediawiki/core@master] styles: Settle on .user-select() mixin

https://gerrit.wikimedia.org/r/1037742

Change #1037742 had a related patch set uploaded (by VolkerE; author: VolkerE):

[mediawiki/core@master] styles: Settle on .user-select() mixin

https://gerrit.wikimedia.org/r/1037742

Upstream is now at https://issues.chromium.org/issues/40578413

Change #1035561 merged by jenkins-bot:

[mediawiki/core@master] mediawiki.less: Remove outdated basic browser support workarounds

https://gerrit.wikimedia.org/r/1035561

Change #1035558 merged by jenkins-bot:

[oojs/ui@master] styles, demos: Remove further IE 11 CSS workarounds

https://gerrit.wikimedia.org/r/1035558

Change #1035556 merged by jenkins-bot:

[oojs/ui@master] Element: Remove IE specific `rtlScrollType` handling

https://gerrit.wikimedia.org/r/1035556

Change #1035554 merged by jenkins-bot:

[oojs/ui@master] styles: Remove IE 11 placeholder workaround

https://gerrit.wikimedia.org/r/1035554

Change #1035561 had a related patch set uploaded (by VolkerE; author: VolkerE):

[mediawiki/core@master] mediawiki.less: Remove outdated basic browser support workarounds

https://gerrit.wikimedia.org/r/1035561

Change #1035558 had a related patch set uploaded (by VolkerE; author: VolkerE):

[oojs/ui@master] styles, demos: Remove further IE 11 CSS workarounds

https://gerrit.wikimedia.org/r/1035558

Change #1035556 had a related patch set uploaded (by VolkerE; author: VolkerE):

[oojs/ui@master] Element: Remove IE specific `rtlScrollType` handling

https://gerrit.wikimedia.org/r/1035556

Change #1035554 had a related patch set uploaded (by VolkerE; author: VolkerE):

[oojs/ui@master] styles: Remove IE 11 placeholder workaround

https://gerrit.wikimedia.org/r/1035554

https://gerrit.wikimedia.org/r/c/mediawiki/core/+/1035549/ was actually part of this cleanup.

Test wiki on Patch demo by ESanders (WMF) using patch(es) linked to this task was deleted:

The reproduction also needs Vector-2022 skin enabled with sticky header, along with “Place categories above all other content.” gadget enabled too.
Firefox well opens the select downward: no issue. I don’t see interest to opening the select upward, so I would turn

if ( textPos.y < maxListHeight + offset + 1 ) {
	// The list might extend beyond the upper border of the page. Let's avoid that by placing it
	// below the input text field.
	nt = this.text.offsetHeight + offset + 1;
	if ( this.engineName ) this.engineSelector.style.top = this.text.offsetHeight + 'px';
} else {
	nt = -listh - offset - 1;
	if ( this.engineName ) this.engineSelector.style.top = -( offset + 1 ) + 'px';
}

into

// The list might extend beyond the upper border of the page. Let's avoid that by placing it below the input text field.
nt = this.text.offsetHeight + offset + 1;
if ( this.engineName ) this.engineSelector.style.top = this.text.offsetHeight + 'px';

I'm still not sure what causes the issue you are experiencing but I assume it's some local / custom add-on or extension in your web browser.
Glad it works for you when logging in!

Change #1017120 merged by jenkins-bot:

[mediawiki/core@master] Update OOUI to v0.49.1

https://gerrit.wikimedia.org/r/1017120