When you are attacking a monster and if you want to move or cancel your attack you can't until your skill/auto-attack is finished. Sometimes i missclick on a monster and can't cancel my mistake so i aggro a pack of monsters and pray for my survive.
I think movements have more higher priority than attack.
For exemple: try tu use auto-attack with an archer and when your arrow is up to go, try to move forward. The arrow will be launch and you will move after, not at the moment on you press W (or Z for azerty) See the result.