Steps to reproduce:

1. about:preferences > General
2. Open Advanced... in Fonts and Colors section

Actual Results:
Immediately after the dialog is displayed, the lower part of the dialog is cut-off.
See attached screenshot, OK Cancel Help button are cut-off

Expected Results:
Should not

Regression window:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=8aa1b80a8c6b66cccc2e37080fbf6197eee8d801&tochange=1716229005d8b97f305bd663b8bc8d3fec4a4f3a

I can reproduce the bug in Windows 10 (with a smaller cut-off) but not on Linux and macOS where the buttons look normal.

This cut-off is happening because the font lists load dynamically. When the
font builder code is running, it removes the menupopup and there is no content.
Then we measure the height of the dialog and its many menulists and persist that.
But the heights of these menulists is smaller (depending on Windows font size
and scaling settings) when they have no content than when they have text in them,
and so when the menulists fill up the height changes, leading to the cut-off.

To fix this, set a minimum height when the menulists are empty, that either matches
or exceeds the min-height they get when they have content.

Backed out 3 changesets (Bug 1704113, Bug 1705330, Bug 1708148) for causing bc failures in browser_username_select_dialog.js and mochitest failures in test_modal_select.html.
https://hg.mozilla.org/integration/autoland/rev/3d2225b699c12f33140428b4be4bf8362c370df7

https://hg.mozilla.org/mozilla-central/rev/1bd69e1e04ce

Alice, could you confirm if this is fixed for you in today's nightlies?

(In reply to :Gijs (he/him) from comment #8)

Alice, could you confirm if this is fixed for you in today's nightlies?

The problem is no longer reproduced on Nightly90.0a1(20210430092829) Windows10.

(In reply to Alice0775 White from comment #9)

(In reply to :Gijs (he/him) from comment #8)

Alice, could you confirm if this is fixed for you in today's nightlies?

The problem is no longer reproduced on Nightly90.0a1(20210430092829) Windows10.

Perfect, thank you!

Comment on attachment 9219071 [details]
Bug 1704113 - fix fonts subdialog cut-off in the preferences, r?jaws

Beta/Release Uplift Approval Request

• User impact if declined: The font dialog is hard or impossible to use (depending on text size + DPI)
• Is this code covered by automated tests?: Yes
• Has the fix been verified in Nightly?: Yes
• Needs manual test from QE?: No
• If yes, steps to reproduce:
• List of other uplifts needed: n/a
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): We're using some CSS to ensure height calculations work more reliably.
• String changes made/needed: Nope

Comment on attachment 9219071 [details]
Bug 1704113 - fix fonts subdialog cut-off in the preferences, r?jaws

Approved for 89 beta 7, thanks.

https://hg.mozilla.org/releases/mozilla-beta/rev/4bd7350a7cdf