How do I get xkb settings to work via xwayland? They work fine when used on Wayland-native applications

Question

My custom keyboard mapping only works through xdg_shell applications, not through xwayland. I have configured Sway to show in the title bar what's running via xwayland and what's not, using for_window [tiling] title_format "%title — $shell".

My question: Is this a bug in Sway, XWayland, or is there something I can do to my config to make it work?

Versions:

• System: Ubuntu 20.10
• Sway: v1.5
• Xwayland: package version: 2:1.20.10-2

I created an xkb keymap using xmodmap under X. Example content:

add    mod3 = Mode_switch
keycode  40 = e E U2203 U2203

(keycode 40 because I run Dvorak)

This makes my Right Alt key Mode_Switch (Mod3), and makes Mod3+e write "∃" ("there exists").

I dumped my xkb using: xkbcomp -xkb :0 test.xkb. The relevant mapping in that file is:

   key <AC03> {
        type[group1]= "ALPHABETIC",
        type[group2]= "ALPHABETIC",
        type[group3]= "ONE_LEVEL",
        type[group4]= "ONE_LEVEL",
        symbols[Group1]= [               e,               E ],
        symbols[Group2]= [           U2203,           U2203 ],
        symbols[Group3]= [        NoSymbol ],
        symbols[Group4]= [        NoSymbol ]
    };

Then in Sway I use xkb_file test.xkb to load this keyboard layout. This works fine in apps that go via xdg_shell, but not ones that go via xwayland. Via xwayland it becomes nothing.

Examples X programs where it doesn't work:

• xterm
• google chrome (started with default parameters)

Example native wayland:

• gnome-terminal
• google chrome started with -enable-features=UseOzonePlatform -ozone-platform=wayland

I'm also comparing the output of xev -event keyboard no wev -f wl_keyboard.

In both X and Wayland I see the Mode_Switch keypress, but the following e key becomes keysym 0x0, NoSymbol. In wev I see the symbol being written just fine. See below for exact outpu.

xev -event keyboard
KeyPress event, serial 28, synthetic NO, window 0xc00001,
    root 0x525, subw 0x0, time 371977355, (63,73), root:(3107,635),
    state 0x0, keycode 66 (keysym 0xff7e, Mode_switch), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyPress event, serial 28, synthetic NO, window 0xc00001,
    root 0x525, subw 0x0, time 371977635, (63,73), root:(3107,635),
    state 0x4000, keycode 40 (keysym 0x0, NoSymbol), same_screen YES,
    XLookupString gives 0 bytes: 
    XmbLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 28, synthetic NO, window 0xc00001,
    root 0x525, subw 0x0, time 371977787, (63,73), root:(3107,635),
    state 0x4000, keycode 40 (keysym 0x0, NoSymbol), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False

KeyRelease event, serial 28, synthetic NO, window 0xc00001,
    root 0x525, subw 0x0, time 371981259, (63,73), root:(3107,635),
    state 0x4000, keycode 66 (keysym 0xff7e, Mode_switch), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False

wev -f wl_keyboard
[14:     wl_keyboard] modifiers: serial: 0; group: 0
                      depressed: 00000000
                      latched: 00000000
                      locked: 00000000
[14:     wl_keyboard] key: serial: 120526; time: 372129172; key: 66; state: 1 (pressed)
                      sym: Mode_switch  (65406), utf8: ''
[14:     wl_keyboard] modifiers: serial: 1; group: 0
                      depressed: 00000000
                      latched: 00000000
                      locked: 00000000
[14:     wl_keyboard] key: serial: 120528; time: 372129674; key: 40; state: 1 (pressed)
                      sym: U2203        (16785923), utf8: '\u2203'
[14:     wl_keyboard] key: serial: 120529; time: 372129756; key: 40; state: 0 (released)
                      sym: U2203        (16785923), utf8: ''
[14:     wl_keyboard] key: serial: 120530; time: 372130300; key: 66; state: 0 (released)
                      sym: Mode_switch  (65406), utf8: ''
[14:     wl_keyboard] modifiers: serial: 0; group: 0
                      depressed: 00000000
                      latched: 00000000
                      locked: 00000000