Setting the cursor to ArrowCursor when calling XGrabPointer() prevents the Screen savers from blanking the mouse cursor.

I don't know why this has been done in the first place, but I couldn't see any negative effect by setting it to None. Now the mouse cursor even changes to the IBeam again when over the password field, which I find more intuitive.

Configure a Screen saver in systemsettings and wait for it to kick in (or lock the screen manually).
Previously (since 4.10) the mouse cursor stayed visible, now it is blanked like it was the case before 4.10.
Moving the mouse/pressing a key (to quit the Screen saver) makes the mouse cursor appear again as it should, regardless of whether the screen is locked or not.