strange...I tested with my current game and stencyl version 8868.
when updating
-if control "left" is down
-set "cursor" to x of mouse etc...
works fine, even when testing in mozilla, as where opening your game in mozilla the cursor behaves badly when F is down. If you can't fix it can't you just have the player to take out the gun when F is pressed (and he is on a ladder etc.) and when F is pressed again, he puts it back? It sucks not to be able to have it exactly as you want it, but if I as a player would have to choose, Id rather have to press F again than having a cursor that is almost impossible to aim with.
btw with the test game in your previous post, which button is uspposed to be pressed down to make the cursor laggy?