Tablet input should be associated with tablet output for tablets with an integrated screen

I am testing the now-merged !546 (merged) on a Wacom Cintiq 21UX, and Weston doesn’t puts the tablet pointer on the expected (secondary) output, when the main output is my laptop’s screen.

Mutter/gnome-shell does the correct mapping, but I have no idea how this association is described by the kernel.

Ping @mol.