How to make modifier keys lockable? (like Caps Lock for Ctrl or Alt)

Question

Playing around with the non-modal text editor ne, I found myself wondering if one could simulate a modal interface (like vim has) by having something like "Ctrl Lock" or "Alt Lock" (analogous to Caps Lock) and thus a "ctrl mode"/"alt mode". I would like to try this, but I am not sure how to make it happen.
My best guess is that this could be accomplished via XKB, but its documentation looks pretty intimidating. Is anyone here proficient in XKB and could point me in the right direction?
Other ideas:
– Is this something that a terminal emulator could do? Is there already one that can do it?
– Or a shell?
– For console applications, would it be possible to have a small wrapper that did nothing but intercept keyboard input and just pass everything through to the wrapped process, except that it would take into account the desired lockable modifier key? (I am thinking of tput here, but from my first search it appears that it doesn't do this sort of thing.)

Answer 1

@Trilby: Thank you for the elaborate input!
@drcouzelis: "sticky keys" normally means that modifiers remain active only until the next keypress, while I want a way to lock them in active state for an arbitrary number of keypresses. But still, "sticky keys" was a worthwile input for the search engine.
The program xkbset, which can be used to set XKB options on the fly, has a function that comes very close to what I desire:
$ xkbset h
Usage: To set or unset various options:
xkbset <options>
where <options> may be all or any of (the '-' switches the feature off,
otherwise it is switched on):
To switch sticky keys on or off, and optionally set or reset:
() two keys pressed at the same time stops sticky keys;
() a modifier pressed twice will be locked:
[-]{sticky|st} [[-]twokey|[-]latchlock]...
I cannot select specific modiers to be affected with xkbset, but that would be fine with me. Sadly, it didn't completely work for me:
– It doesn't affect Alt at all for me.
– The effect seems to go away as soon as I change the window. (Maybe I need to invoke xkbset from .xinitrc for its effects to persist throughout the session? I will try that.)
So I went on and tried to configure XKB manually. I got up to a certain point without studying the documentation in-depth, but now I have landed on an error message that seems to suggest to me that I would have to get to a better understanding about the layout files before I could understand it. Here is what I did:
1. Got my current XKB configuration:
xkbcomp :0 output.xkb
2. Editet output.xkb to include the following at the end of the xkb_compatibility section:
//Hoping this makes the ^-key into a Ctrl-Lock key:
interpret TLDE {
action= LockMods(modifiers=Control);
3. Then I recompiled the configuration back into the running X server:
$ xkbcomp out.xkb :0
Warning: Compat map for group 2 redefined
Using new definition
Warning: Compat map for group 3 redefined
Using new definition
Warning: Compat map for group 4 redefined
Using new definition
Error: Couldn't lookup keysym
Symbol interpretation ignored
Warning: No symbols defined for <AB11> (keycode 97)
Warning: No symbols defined for <JPCM> (keycode 103)
Warning: No symbols defined for <I120> (keycode 120)
Warning: No symbols defined for <AE13> (keycode 132)
Warning: No symbols defined for <I149> (keycode 149)
Warning: No symbols defined for <I154> (keycode 154)
Warning: No symbols defined for <I168> (keycode 168)
Warning: No symbols defined for <I178> (keycode 178)
Warning: No symbols defined for <I183> (keycode 183)
Warning: No symbols defined for <I184> (keycode 184)
Warning: No symbols defined for <FK19> (keycode 197)
Warning: No symbols defined for <FK24> (keycode 202)
Warning: No symbols defined for <I217> (keycode 217)
Warning: No symbols defined for <I219> (keycode 219)
Warning: No symbols defined for <I221> (keycode 221)
Warning: No symbols defined for <I222> (keycode 222)
Warning: No symbols defined for <I230> (keycode 230)
Warning: No symbols defined for <I247> (keycode 247)
Warning: No symbols defined for <I248> (keycode 248)
Warning: No symbols defined for <I249> (keycode 249)
Warning: No symbols defined for <I250> (keycode 250)
Warning: No symbols defined for <I251> (keycode 251)
Warning: No symbols defined for <I252> (keycode 252)
Warning: No symbols defined for <I253> (keycode 253)
The "Error" part is where I am currently stuck. It tells me that my changes were ignored (I no that it's about them because it goes away when I comment them out), but I don't yet understand why.