Alt-Key not working on Linux


Hey :)

I'm on Arch LInux with Gnome 3.18.

In my mouseDown(), ... functions the Alt-key never gets set as a modifier. I tried to find the error and after searching in the linux windowing code I think I found it. After the Alt-key was pressed and a keyboard event was send everything is fine and the flag for the Alt-key is set properly. But as soon as you move the mouse it gets unset.

The difference between the two cases is that after a keyboard event updateKeyModifiersFromSym() in juce_linux_Windowing.cpp is called and after a mouse event the function updateKeyModifiers() is called.

The updateKeyModifiers() triggered the Alt-key when I pressed Meta on my keyboard. It checks for a flag specified by Keys::AltMask that is set in updateModifierMappings(). The keycode for Alt gets determined by XKeysymToKeycode(display, XK_Alt_L), and this seems to be the source of the whole error. On my system that function call returns the same keycode for XK_Alt_L and XK_Meta_L. Based on that error also the value for Keys::AltMask gets set wrong. With my configuration everything worked correct when I set Keys::AltMask manually to 8, but I'm not sure how to make this work in a dynamic way with false output from XKeysymToKeycode().

(I'm also not 100% sure if the bitshifting "if (mapping->modifiermap [i << 1] == altLeftCode)" is correct that way, according to it should be something like modifiermap[i * mapping->max_keypermod])

Already lots of thanks in advance :)


Could anyone reproduce this issue? Or is this a problem with my setup?


I’ve encountered the exact same issue and traced it back to the exact same location. I’ve worked around it in our code but I don’t know enough of X programming to fix it properly.



I’ve spent some time fixing this. The root problem is indeed how the Modifier mask for the alt key is determined. The code is wrong.

Attached is a patch that fixes the problem.

JUCE_alt_key_windows.txt (1.9 KB)