bug(linux): Can't type AltGr characters with onboard

It's not possible to type any key combination with AltGr with the Onboard onscreen keyboard. The characters on the keyboard are displayed correctly, but pressing AltGr immediately puts AltGr in the locked state (AltGr with red background). The Alt key is also marked as pressed (gray background). Pressing any other key in that state does nothing.

When switching to e.g. Khmer Angkor when onboard is open and pressing AltGr on the physical keyboard highlights the left Alt key on the onscreen keyboard.