By default Brave (and other Chrome browsers) have headerbars and enabling it adds a double headerbar though it removes it for other applications.
This is on Pop 20.04 LTS which may void out this bug report which is fine.
gnome-shell --version
GNOME Shell 3.34.3
Just pulled the shell from git a few minutes ago.
This should be fixed by 0ad8552
It appears that window titles are now show up for applications where they didn't previously. Firefox for instance has a title bar now even with Show Window Titles disabled.
Temporary fix for now would be to disable the title bar in Firefox's Customize view
Disabling title bar in firefox "Customize" options does not work on the 20.04 beta
I would like to confirm what Jeroen7s said. Disabling the title bar in firefox "customize" in 20.04 doesn't work.
+1 basically this options doesn't do anything.

Am glad to help in any way. Trying to work through this, for now: https://joshtronic.com/2017/07/26/hide-title-bars-in-gnome-shell/
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Disabling title bar in firefox "Customize" options does not work on the 20.04 beta