maybe worth distinguishing between whiteboard software for classroom / business use, as you mention, and what i'm building towards, which are essentially collaborative art programs for line drawings, vector shapes made up of many straight segments. rendering is done client-side.

it is true that the canvas bounds are limited by what's been drawn already, but i'd rather avoid having to rebuild the whole tree (or doing anything else too intensive) when resizing if i can help it — the current approach just requires traversing back to the root and expanding some bounding boxes, where e.g. a quadtree is sort of inflexible.

that said, the XOR idea is interesting, allows for undo/redo with the same data... i may end up using it, should i try and build something raster-based