User Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/114.0.0.0 Safari/537.36

Steps to reproduce:

Disabled "Settings -> Hide System Window Title Bar"

System / desktop is KDE neon (Ubuntu 22.04 based)

Actual results:

The system's default window title bar is shown (as expected) and the search text box, the extension buttons and the Hamburger icon to trigger the menu move into a row below the window's title bar.
However, the Hamburger icon is non-responsive, clicking it does not open the menu any more.
It works if Thunderbird's custom window title bar is active.

Expected results:

The Hamburger icon should open Thunderbird's menu also if the system's default window title bar is being used.

We had another user test this and it worked for them. This is with KDE Plasma 5.27.6 / Wayland.

I can look into this.

In my case it's KDE Plasma 5.27.6, but X11.