I think a much simpler idea would to have a onmousedelta event which fires whenever the mouse moves and that tab has focus (including outside window boundary) and includes a .deltaX and .deltaY event property.
That way, the user has full control over their mouse and you can also develop fps games.
It's unlikely that the operating system's standard mouse events will continue to report mouse movement when the cursor runs into the edge of the screen. Plus it would be very distracting to have a mouse cursor running around outside the active window.
What happens when the user moves the cursor outside the window, and then clicks? Or moves the cursor into a hot corner of the screen, activating something like a screen saver?
Deltas are necessary but are only part of the solution.
That way, the user has full control over their mouse and you can also develop fps games.