If your iPad’s keyboard does not work when using a mouse on iPadOS 13
iPadOS 13 includes the ability to use an external pointing device, such as a mouse, with an iPad. Within iPad’s Accessibility settings includes a feature called Mouse Keys. Mouse Keys allows the AssistiveTouch pointer to be controlled using the keyboard number pad. Most often iPad will be paired with a Smart Keyboard which does not include a number pad. Mouse Keys will default the pointer control’s keys to the following:
7: Upper-Left
8: Up
9: Upper-Right
U: Left
O: Right
J: Lower-Left
K: Down
L: Lower-Right
With Mouse Keys enabled the keys mentioned above will no longer be able to type their associated character, as they will now be used to position iPad’s pointer.
Mouse Keys can be managed in:
Settings > Accessibility > Touch > AssistiveTouch > Mouse Keys