Starting to learn C I took the opportunity to glance over the source code since I had the same problem that copying the QR code's content doesn't work. I found that the content is being copied into the primary selection clipboard (line 503) which is usually pasted with a middle mouse click. I don't know of a corresponding keyboard shortcut, so I connected a mouse and lo and behold, pressing the middle mouse button (or rather the scroll wheel) pastes the content!
I will propose a patch when I have the time to figure out how to do that (unless someone of the developers sees this before and is faster).
Edit: Apparently shift + insert is a common shortcut to paste the primary selection, but it doesn't work with my keyboard. Also there is no way to reach that shortcut with squeekboard's terminal layout. For now maybe you can find a way to map a custom keyboard shortcut or use wl-clipboard (wl-paste --primary), that should work too.
I will propose a patch when I have the time to figure out how to do that (unless someone of the developers sees this before and is faster).
Edit: Apparently shift + insert is a common shortcut to paste the primary selection, but it doesn't work with my keyboard. Also there is no way to reach that shortcut with squeekboard's terminal layout. For now maybe you can find a way to map a custom keyboard shortcut or use wl-clipboard (wl-paste --primary), that should work too.