Yeah, unfortunately it seems to be a limitation of the browser where it doesn't send the mouse-up signal if the cursor is off the canvas. Using full-screen can help with that, but it doesn't fix the issue with multi-monitor setups.
The download version doesn't have that limitation, and you can let go of the mouse button off of the window and still have it register. I know it's not as convenient, so I have both the browser and download versions available so people can use whatever works best for them.