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.
It’s been a thing I personally have been wondering why this is how it is for a while. Personally I like most of the GNOME stuff, but this decision has always stood out as odd.
But then again I almost always use ctrl+w or alt-f4 to close apps, so I am mostly unaffected.