That is because "mouse is down" condition triggers continuously as long as the mouse button is down, thus starting the animation over and over and over each tick.
That's exactly what I want.
I don't understand the following: when I set animation speed to 10 or 20, I only see 1 frame.
Animation speed set at 50, it jumps, seemingly random, from the 1st to the 2nd frame. In a non-constant way. (completely destroying the illusion of animation/movement)
I wish to achieve the following result:
As long as mouse button left is down, animation of 2 frames starts (gun-shooting animation).
Mouse button released: frames of gun-shooting animation are no longer seen.