Windowed Mode- Cursor doesn't lock onto screen

It seems that when I play LoL, the cursor doesn't stay within the game boundaries, which makes it super hard to scroll around the map. Don't really know what's going, but another thing to note is that sometimes the cursor changes to the regular Windows cursor instead of the league cursor. Anybody got a solution?

And before anybody tells me to alt-tab and alt-tab back in, it doesn't work. At least for me, anyways.