Ohh, I think I know what it is. When you click off a game, you lose "focus," and don't have control until you click back in, and when you open a new window (or, in some cases, even do simple things like change your volume) while pressing keys, those inputs are remembered and stretched until you focus back into the game and press the key again and then let go. At least I think. The easy fix for this would be to play in fullscreen.
If it's not that, not sure what it could be, but what was the issue where the boss doesn't reset?