The main aim for this commit is to fix an exception that is thrown when GLUT without SDL is used. I haven't used pointer lock yet, so I didn't notice the issue here.

"This is not correct though, we need to get the GLUT mouse position, here, don't we?"
I'm not 100% positive about this, but from looking at it, this method should update the Browser.mouse* fields (absolute+delta) no matter what API/framework is used. 

"It seems like SDL should just store mouseX and Y on browser, and so should the others."
Yes, but I'm not sure how SDL expects it's mouse coordinates. Maybe there needs to be some offest or other factor applied.
However we've already got Browser.mouseX/Y so you're right and I think the following change should patch it up for non SDL cases.

line: 437

        	Browser.mouseX += Browser.mouseMovementX;
        	Browser.mouseY += Browser.mouseMovementY;