Steps to reproduce:
1) Switch to windowed mode.
2) Start a large level that allows scrolling.
3) Hit the "Release Mouse" hotkey.
4) Move the mouse outside the actual game window.
If you now move to one of the sides, where the level can scroll to, it in fact does scroll in that direction, even when the cursor is completely outside the game window.
The indended behavior (as far as I am concerned) is to scroll only when the mouse is caught within the game window.