Just a note from the discussion that this PR may not be relevant at this point, but I just wanted to get this put together in case it is

Tested these commits on Gentoo with Hyrland. This resolves the problem on the fullscreen mode.

I feel that we should not be changing Ghostty's behavior with respect to gtk-titlebar and window-decoration until we update the documentation surrounding these configuration entries (perhaps adding/modifying options in the process). In particular here are the last two lines of the gtk-titlebar documentation that indicate that gtk-titlebar doesn't have any effect after initial window creation. This patch also does not take into account situations when window-decoration might affect the display of the titlebar.

Everyone seems to want to modify the behavior to suit their particular interpretation of what gtk-titlebar should mean and how it interacts with window-decoration. We need to put more work first in clarifying the documentation so that everyone agrees on the interpretation and it's possible to clearly specify a user's wishes.

For example, there is no documented way currently to turn off window decorations (rounded corners and drop shadows) and keep the GTK titlebar.