some keys not seen in xev(1) when pressed in combination with both Alt_L and Alt_R

[Michai Ramakers <m.ramakers%gmail.com@localhost>]

Hello,

first of all, I am not well versed in xmodmap(1) or general key
handling whatsoever.

I would like to use specific key combinations in the window manager I
use ('herbstluftwm' - but I think this is irrelevant in this post),
but failed. One of those key combinations is 'left Alt + right Alt +
h'.

After starting X11 without window manager but only an xterm instead,
and running xev(1), I noticed that
  1) either Alt-key shows a pressed/released events,
  2) the 'h' key in combination with either Alt-key shows events,
  3) the 'h' key in combination with both Alt-keys at once does not show events

Pressing more letter-keys while both Alt-keys were pressed, showed
that on this particular system (Dell Latitude C840), keys 'g', 'h',
'b' and 'n' out of 'a'-'z' did not generate events with both Alt_L and
Alt-R pressed.

Repeating this test with an external USB keyboard showed the same
results. Repeating this test on another system (VIA Epia something)
showed that about 50% of the letter-keys would not generate events in
combination with both Alt-keys pressed.

As far as I could tell, all letter-keys generate events when used with
any combination of left-hand Ctrl, Alt and Shift. So that's what I'll
use for now, then.

Does anyone know an explanation for this behaviour? The reason I would
like to use both Alt-keys simultaneously, is that I can assign those
particular combinations to the window manager, leaving Alt+letterkey
combinations available for applications.

Thanks,
Michai