Oh! You can move them by clicking and dragging on the edges! Before, I moved them by resizing them (by clicking and dragging the square in the middle of an edge). I didn't know you could click anywhere else on an edge. Oh well!
I was considering changing the mouse pointer do a dragging hand icon when you're over draggable things, but the dragging hand icon I've got is frankly a bit crap.
I thought that the square in the corner of a box was supposed to move the box, but that just moved the view of the canvas.
Why does the corner square move the canvas view? Wouldn't it be better if you did that by holding Ctrl and dragging around with the mouse?
Aha, the corner square moving things is a bug (the square is just meant to be a marker of which corner of the frame is which). Ctrl or shift + drag is a good idea. At the moment you can move the canvas by dragging on the beige background, or with the cursor keys. Mouse wheel or -/= zooms.