Issue: GTK Titlebar Theming Regression in Web Browser Titlebars

[mrust1219]

Steps to Reproduce:

Use the latest Cosmic.
Enable the browser titlebar theme (where the application controls its own titlebar).
Open a web browser that uses its own titlebar (e.g., Firefox, Chromium, Brave).
Notice that only the close window button is visible, with the minimize and maximize buttons missing.

Expected Behavior:

When using a browser titlebar, the GTK window titlebar should display all three window control buttons: close, minimize, and maximize.
Actual Behavior:

Only the close button is shown in browsers using their own titlebars (not set by system), while the minimize and maximize buttons are absent. This issue seems to have arisen after a recent update to Cosmic.

Workaround:

The issue can be worked around by switching to system titlebars, which restores the missing buttons, but this is not ideal as users typically prefer the browser titlebar style.

Additional Information:

The issue specifically affects browsers with their own titlebar implementation, and GNOME App theming is enabled in the Appearance section of the Cosmic settings.
This regression occurred after the most recent update.
The problem is specific to GTK-based web browsers like Firefox and Chromium.

System Details:

Arch Linux
Cosmic Desktop Environment (latest)
Affected browsers: Firefox, Chromium, Brave, etc. (or any other browsers that use their own titlebar)

[Quackdoc]

I am noticing this with all gtk apps. Gedit for instance is included. Even with apply to gtk apps disabled.