XKB: modify layout to Multi-group with Mode_switch

Question

I want to create a custom keyboard layout based on my German keyboard layout (de). The idea is to have a two-group layout and have the Caps-Lock to be the Mode_switch key. That remap seems to work according to xev. However, the second group layout does not work. When I press [CAPS]+[H] --> "h" occurs ("`" expected).

Here is my symbols file called de_ext:

default
xkb_symbols "basic" {

    include "de"

    name[Group1]="de";
    name[Group2]="de2";

    // the H button should become accent grave when in second group
    override key <AC06> {
        type[Group2]="ALPHABETIC",
        symbols[Group2]= [           grave, grave ]
    };

    override key <CAPS> {         [     Mode_switch, Caps_Lock ] };

};

Edit 1:

@quixotic is right, this actually works when set directly. I also tried copying the de_ext file to /usr/share/X11/xkb/ and then running setxkbmap de_ext which also works.

However, it does not work when adding the layout to /usr/share/X11/xkb/rules/evdev.xml with

<layout>
  <configItem>
    <name>de_ext</name>
    <shortDescription>xy</shortDescription>
    <description>German extended</description>
    <languageList>
      <iso639Id>ger</iso639Id>
    </languageList>
  </configItem>
</layout>

Although the new layout appears in my layout selection on my Ubuntu 17.04 taskbar, when I select it, the CAPS-Lock button has a different behavior as explained above. What works is that CAPS-Lock is the Mode_switch (I checked with xev), however, the second group doesn't seem to be selected because when pressing [CAPS]+[H] it doesn't work. Any idea what the difference could be when using the Ubuntu/Gnome Layout Switcher?

Edit 2: When calling xkbcomp $DISPLAY broken.xkb, these are the things I noticed:

1. The name of my xkb_symbols is: xkb_symbols "pc+prg+de:2+us:3+inet(evdev)"

2. My Layout has three groups instead of two (I noticed that even my default German layout gets two layouts although only one is defined in symbols/de)

   name[group1]="German";
   name[group2]="German";
   name[group3]="English (US)";
   

3. Almost all keys get three groups / layouts, whereas the second is the same as the first and the third one is an English keyboard layout. My own changes to the letters are not included.

   key <AC01> {
       type[group1]= "FOUR_LEVEL_ALPHABETIC",
       type[group2]= "FOUR_LEVEL_ALPHABETIC",
       type[group3]= "ALPHABETIC",
       symbols[Group1]= [               a,               A,              ae,              AE ],
       symbols[Group2]= [               a,               A,              ae,              AE ],
       symbols[Group3]= [               a,               A ]
   };
   

4. Here is what I get for the letter H

   key <AC06> {
       type[group1]= "FOUR_LEVEL_ALPHABETIC",
       type[group2]= "ALPHABETIC",
       type[group3]= "ALPHABETIC",
       symbols[Group1]= [               h,               H,         hstroke,         Hstroke ],
       symbols[Group2]= [               h,               H ],
       symbols[Group3]= [               h,               H ]
   };
   

5. My change for the Caps-Lock button is included without modifications

What I assume: there must be some rule in the default (ubuntu?) xkb configuration which modifies all symbols to include additional layouts as an additional group. Is there any way I can stop that rule for my xkb symbol to fire?

Answer 1

I finally found the solution. In the file /usr/share/X11/xkb/rules/evdev there is a rule

! model     layout[2]   =   symbols
  *         *           =   +%l[2]%(v[2]):2

Which causes to overwrite my second group with the default layout of my keyboard. I could fix this by adding the following line (where de is my base keyboard layout):

! model     layout[2]   =   symbols
  *         de          =   +de

Thank you @quixotic to help me analyzing the problem