Navigation controls overlap other processes

I am following a tutorial, have followed the tutor to the letter and have used the exact settings and file that he uses, but when I am trying to paint vertex colours and use ■■■■ + lmb to erase, it partially paints out but also pans across the level as if both commands are trying to take precedent. Is this a bug as I cannot seem to paint/erase without panning across the asset?

massively frustrating