[Bug 1939182] [NEW] Title bar is black and unrendered when turned off

[David Yang]

Public bug reported:

After creating the value browser.tabs.drawInTitlebar and setting it to
true, the tabs are moved up to a bar with the window controls, but the
title bar is black and still present. This black bar is still draggable.
Also, enabling the flag makes firefox render strangely on startup. These
issues only happen in fullscreen mode, windowed mode does not have the
black bar, although windowed mode does have a blank spot in the top left
corner which I don't know is intentional or not.

Mint 20.2, but I'm using the ubuntu firefox next PPA at
https://launchpad.net/~mozillateam/+archive/ubuntu/firefox-next,
cinnamon 5.0.5. 750M GPU, Driver Version: 418.113

An imgur link provided for illustration: https://imgur.com/a/Yj9RFIL

** Affects: firefox (Ubuntu)
     Importance: Undecided
         Status: New

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Title:
  Title bar is black and unrendered when turned off

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