Perhaps this is somehow related to the convertible lid hinge. Ordinarily when the laptop is in "tablet" mode (hinge open > 180 degrees), the keyboard and touchpad are disabled. Even more reliably than using an external device, switching into and then back out of tablet mode by opening/closing the hinge seems to restore keyboard/touchpad functionality. Not sure why they appear to work until the gdm screen though, or why sometimes using an external device causes them to work again.
Perhaps this is somehow related to the convertible lid hinge. Ordinarily when the laptop is in "tablet" mode (hinge open > 180 degrees), the keyboard and touchpad are disabled. Even more reliably than using an external device, switching into and then back out of tablet mode by opening/closing the hinge seems to restore keyboard/touchpad functionality. Not sure why they appear to work until the gdm screen though, or why sometimes using an external device causes them to work again.