On Windows, KDE Plasma, and likely many other Desktops, if a window is fullscreen maximized and you push the mouse to the top edge and click, it will close.
Chrome-ium actually fixed that in their builtin buttons to work the same.
Not on GNOME because there is a panel at the top, lol.
But also not when using GTK apps on these desktops, where it should work. Instead you need a lot of precision, for no reason.
An easy fix would be to expand the actual clickbox further, not only around the (oversized) close button circle, but to the edge of the screen window.
This would make Thunderbird, Firefox, etc. closable likel any other normal app. ;D
If you support this, leave a like on the issue. And lets hope this doesnt get closed because of whatever…
Edit
This is about maximized, not fullscreen windows. But also about those.
And the request is to expand the clickbox to the corner of the window, not of the screen.
Yes I thought about that exact argument. They oversize their panels on purpose, there is tons of other space to click on, which is also way less risky, that next to the close button.
And this expansion is about all decoration buttons of course.