Here is another little bit of info for working around your canvas.
The border of the screen is not the end. The canvas of MyPaint grows as you expand your painting, only the memory of your computer sets a limit. Pan around your painting area using the Middle mouse button (MMB) or by holding the Space bar down. You can zoom in and out using the + and - keys.
A common thing for painters to do is to look at their painting from different angles (this is a time-honoured technique for finding flaws in anatomical drawings and portraits for example). MyPaint allows you to rotate your view of the painting as well as mirror it to either side (see the View menu). If you get lost, use Reset (Zoom, Rotation, Mirror) from the View menu.
One thing to remember is that these commands allows you to view your canvas differently but they do not modify the painting itself. So if you e.g. save a rotated picture it will still appear un-rotated when you load it later. If you want to actually rotate, mirror or enlarge your painting, use GIMP.
basic MyPaint instrucs