After pressing AltGr, the corresponding characters are not displayed on the buttons #11

Basic information

TDE version: R14.0.10 [DEVELOPMENT]
APP version: 0.4.8
Distribution: Jessie/Stretch
System: Linux 4.9.0-14-amd64
Hardware: x86_64
Compiler: GNU 4.9.2

Description

On the Czech keyboard (and probably others), AltGr functions as a modifier, similar to Shift. For example, AltGr+Q writes \, AltGr+W writes |, AltGr+X writes #, AltGr+C writes &,… Kvkbd including all current patches writes the expected characters, but after pressing AltGr on the virtual keyboard is not rendered corresponding characters on the buttons.

Steps to reproduce

Add the Czech keyboard layout and select it as active.
Press on kvkbd right Alt (AltrGr).
Observe that the characters on the buttons remained unchanged.
Press X – you will see that it will be written #.

Displaying AltGr characters would be a nice-to-have feature but that result is consistent with the current source code.
kvkbd doesn't generate the print characters but takes the X11 data generated from the keypress to display the character on the key.

AltGr characters are 3rd level characters and are defined in the keyboard symbols files, eg Czech keyboard, probably at /usr/share/X11/xkb/symbols/cz

key <AB02>  { [         x,          X,   numbersign,     Nosymbol ] };

Here, 'X' is the shift character - it just happens to be the same as upper case 'x'.
And AltGr+Shift+x prints '>' because the latin map takes precedence where no symbol is defined.

At the moment, only 1st and 2nd level characters are generated for display on the keys.
In MainWidget::setupText(VButton& v):

	KeySym keysym_l = XkbKeycodeToKeysym(display, keycode, 0, 0);

provides the 1st level, lower, character, and

	KeySym keysym_u = XkbKeycodeToKeysym(display, keycode, 0, 1);

provides the 2nd level, shift, character, with each of those capitalized with upper(),
and it will need something like

	KeySym keysym_a = XkbKeycodeToKeysym(display, keycode, 0, 2);

added to be able to display the 3rd level, AltGr, characters.

And

	KeySym keysym_b = XkbKeycodeToKeysym(display, keycode, 0, 3);

if you want the 4th level, AltGr+Shift, characters displayed as well, for example ß:

key <AC11>  { [   section,     exclam,   apostrophe,       ssharp ] };
                  §            !         '                 ß

I'll be happy to help in any way I can to implement those changes, but it's unlikely I'd be able to make much of a contribution to the programming effort.

Hi @SlavekB, @Ray-V,
this was already on my TODO list after merging #10, in fact I had already started to code this, together with some code restructuring. I will create a PR for testing when finished and you can both help out testing it before we merge it to master.

Fixed by PR #12.