can't add more than 4 layouts to the keymap

[bam80]

libxkbcommon/src/keymap.h

Lines 91 to 104 in 2530f64

	/* This limit is artificially enforced, we do not depend on it any where.
	* The reason it's still here is that the rules file format does not
	* support multiple groups very well, and the rules shipped with
	* xkeyboard-config (see rules/evdev) depend on this limit extensively.
	* So just lifting this limit would cause problems for people who will use
	* more than 4 layouts.
	* TODO: Fix the group index syntax in the rules format, preferably in a
	* backwards compatible way.
	* See e.g. https://bugs.freedesktop.org/show_bug.cgi?id=14372
	* Note: A limit on the number of groups we *do* depend on is imposed by
	* the size of the xkb_layout_mask_t type (32). This is more than enough
	* though.
	*/
	#define XKB_MAX_GROUPS 4

This limit is artificially enforced

It's not documented anywhere, which is an issue per se I already faced, and also it might be useful to add more layouts for example as in our KDE KWin's unit tests (I'll provide the link).
@bluetech

[whot]

Realistically, the main limitation here is XKB. The XKB protocol (the X11 bit) has the limit of 4 groups hardcoded, so if we want to stay compatible to that (and we do, for the forseeable future) extending this is not an option.

A quick peek of the protocol indicates only XkbGetMap has that actually as a sized limit (ktIndex is limited to 4 groups), afaict everything else could make use of at least 256 or (as in that comment) 32 where used as mask. But XkbNumKbdGroups has been #defined to 4 for decades now and I'm sure there is plenty of code out there that hardcodes it. the protocol also defines KB_GROUPMASK as 1-4 groups, so extending this will also cause issues.

[bam80]

Could we make it opt-in maybe? Because right now we already faced with the real issue above due to the limitation, which needs to be solved anyway.

[wismill]

Duplicate of #37, let’s continue the discussion there as it has more info.