Thunderbird dialog too small, can't click Cancel / Send buttons

sometimes thunderbird can’t size this dialog properly where the cancel/send buttons are actually visible…but is that an issue with thunderbird? gtk? kwin? nvidia? x11/xorg? kernel 5.7?

From this I might be able to extrapolate that you are using KDE ?
In that case … you are using a QT desktop. Thunderbird is a GTK application.
It probably has to do with your theming or GTK config, or possibly ‘scaling’.
Do you have hidpi or similar? What is your DPI and/or scaling?
Have you checked with different GTK themes?

I have tried multiple GTK and KDE themes, but it’s still the same…dpi is forced at 96 for readability…

Turning of “force high DPI” in KDE’s font settings has not fixed the issue…