shell-add-wayland-extension=zwp_primary_selection_device_manager_v1 # it isn't obvious why maliit-keyboard needs this
It seems this is necessary due to QtWayland requiring it. However, this commit suggests this may no longer be the case in 6.6.0. I don't know if it's been pulled into the KDE patch collection for 5.15 though.
Now we just need to get Lomiri fully onto Mir 2.x and Qt 5.15 so we can add the capability to ship the OSK in a confined click.