While doing further testing I discovered this version of qt6-webengine has an hardcoded path, leading to this error:
xkbcommon: ERROR: Couldn't find file "rules/evdev" in include paths xkbcommon: ERROR: There are no include paths to search xkbcommon: ERROR: 1 include paths could not be added: xkbcommon: ERROR: /usr/share/X11/xkb xkbcommon: ERROR: Couldn't look up rules 'evdev', model 'pc101', layout 'us,it', variant ',nodeadkeys', options '' [33504:390306304:0115/174930.310788:FATAL:xkb_keyboard_layout_engine.cc(652)] Keymap file failed to load: us,it-,nodeadkeys
And in fact the file src/3rdparty/chromium/ui/events/ozone/layout/xkb/xkb_keyboard_layout_engine.cc includes two instances of /usr/share/X11/xkb.
My patch replaces /usr with %%LOCALBASE%% and then adds this file to an existing REINPLACE_CMD to put there the actual LOCALBASE.
This makes things work, at least with the defaults.
Tested on my machine.