Comment # 2 on bug 977025 from Egbert Eich

Ok, took a quick look:

With:
setxkbmap -layout us,us -variant ',colemak' -option 'grp:win_space_toggle'
a toggle from Group1->Group2 works but not back.

us(colemak) explicitly sets symbols for <SPCE>, us doesn't. win_space_toggle
only sets Group1. This setting is picked up for Group2 only if Group2 isn't set
explicitly. Thus it works with colemak on Group1 and us on Group2 but not the
other way around.

Since us(dvorak) doesn't set <SPCE> either it works both ways with dvorak in
Group1 or Group2.

You'd either have to kill the setting for <SPC> in colemac or change
win_space_toggle to something like:

partial modifier_keys
xkb_symbols "win_space_toggle" {
  key <SPCE> {
    type="PC_SUPER_LEVEL2",
    symbols[Group1]= [ space, ISO_Next_Group ],
    symbols[Group2]= [ space, ISO_Next_Group ]
  };
};

With this information, please open a ticket on bugs.freedesktop.org (product:
xkeyboard-config) - with me (eich@pdx.freedesktop.org) in Cc: - and discuss it
there.
Once a consensus is reached I will add a patch to Leap 42.1.