It is very nice, indeed! Kudos!
There are a couple of things I've noticed, that prolly you know already, taking in the account the alpha state:
- freehand drawing has no continuous mode (it prints the brush in dot mode)
- undo doesn't work with the straight line mode
- all three selection modes don't work (or, if they do, I don't understand how)
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- the box mode selects an area without the red overlay the other modes use, but then it's not excluding the area without the box;
- the polygon and the magic wand selections do have a red overlay but again no exclusion
- all three modes are automatically exited with no selection when you draw something, or make the background non draweable, depending on what tool you try to use on the selection
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- image > clear option doesn't delete the first "batch" of straight lines drawn on the canvas, if after them you switched to a different tool to draw anything else
Probably there is more, but even so, it's a very promising start!