Migrates from the deprecated gdk_device_grab API to the new gdk_seat_grab API.
https://docs.gtk.org/gtk4/migrating-3to4.html#stop-using-gdkdevicemanager
There is code (at least for the keyboard version) that checks if the device has changed. This code had to be removed due to the API changes. Given this functionality has been deprecated by Gtk and is fully removed in Gtk4, I feel this is the right thing to do.
Other
It is unlikely that functionality has been lost given the deprecation.
My sense is that the keyboard device change adjustments were catering to multiple keyboards. In any case, all the code is now run all the time instead of only when a device change was detected. This might have a slight performance hit, but unknown. Though nothing has come up in the performance tests.