xfce4-terminal and mateterminal have the same problem as GNOME terminal.

Ones I've tried (inspired by your suggestion) that don't have the problem are lxterminal, xterm, roxterm and termit. I didn't try an Qt based ones.

I'm going to go out on a limb and suggest the difference might be GTK 2 based systems have the problem and GTK 3 based ones don't.

I think I'll scrap the idea of writing my own and go with lxterminal as it certainly seems that the last thing the world needs is another terminal emulator.