If Weston is using simpledrm, and simpledrm is replaced when Weston is inactive, Weston shows a black screen switching back to it
Weston crashes when simpledrm is replaced when it is active, but when it is inactive, and you try to switch back, Weston doesn't react at all
If it is not possible to handle the primary GPU switching, it should at least abort...
To replicate, on a QEMU vm with the virtio gpu
boot with modprobe.blacklist=virtio_gpu
start Weston on say tty1
switch away from Weston to say tty1
run sudo modprobe virtio-gpu
Try to switch back,