Can't believe I've overlooked that case. I've taken note, will get it fixed in a future patch soon.
This is a known bug indeed. The problem is that it has to do with the webkit wrapper the game runs in, and how it works on Windows; you won't have this problem on, say, OSX (Mac) or in a browser on either OS. The issue is that the "chrome" on Windows (ie. the border surrounding the game window when run in window mode) doesn't fire off mouse events properly, and what practically ends up happening is that the game's code and my plugin won't be told if the mouse left and re-entered the window. I have yet to figure out a way to solve this, because the only "solution" I've found so far is to constantly change the cursor back and forth every frame, which is not good for performance at all. It might be possible if the webkit wrapper MV outputs gets updated, or I find some better hacky solution to fix it. Or we might have to start wrapping our games in a custom webkit wrapper to alleviate this and some other more serious issues. Regardless, you can rest assured I'm aware of it as it bothers me on a daily basis while working on my own game