When drawing, the edges of the main view have an area that act as an
auto-scroll area – if you move your mouse in that area while drawing your page
will scroll automatically which gives you more room to continue drawing.
Sometimes this is undesirable, and the action can be toggled on and off using
this button. Note you can also use the arrow keys while drawing to scroll 1cm
per key click. And holding the spacebar will let you pan the screen by dragging
it with the mouse.