Incorrect initial letterbox size when the interface font size is 13px.

🐞 Bug Report

Reproduction steps

When the GNOME or KDE interface font size is 13px and scaling is at 100%, the initial browser window's letterboxed size is 1400px by 800px instead of 1400px by 900px. The initial window size might be something like 1400px by 899px. This does not occur when a new window is opened after the first one, but it occurs on each start.

Found on Debian testing, and reproduced in a new Debian 12 stable VM. Steps to reproduce: Set font in GNOME Tweaks or Plasma Settings to 13px, and open Tor Browser.

Expected behaviour

Letterboxing borders shouldn't be visible.

Actual behaviour

Letterboxing borders are visible.

Bookkeeping

• Browser version: 14.5
• Browser channel:
  • Release
  • Alpha
  • Nightly
• Distribution method:
  • Installer/archive from torproject.org
  • tor-browser-launcher
  • homebrew
  • other (please specify):
• Operating System:
  • Windows
  • macOS
  • Linux
  • Android
  • Tails
  • Other (please specify):
• Operating System Version: Ubuntu 25.04

Browser UI language

English

Have you modified any of the settings in about:preferences or about:config? If yes, which ones?

No

Do you have any extra extensions installed?

No

Troubleshooting

Does this bug occur in a fresh installation?

Yes, it does

Is this bug new? If it is a regression, in which version of the browser did this bug first appear?

Does this bug occur in the Alpha release channel?

Does this bug occur in Firefox ESR (Desktop only)?

Does this bug occur in Firefox Rapid Release?

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