On mobile I kept opening the whois pixel by accident when dragging. I often tap and hold to initiate a drag because I'm still looking at the art, but when i drag away and let go, it opens the whois thing. I think if you drag a certain screen-space distance away it should cancel the whois pixel lookup.
The heatmap I found too hard to tell where recent pixels were placed. I think at 100% opacity the "cold" pixels should be dark blue instead of their actual color.
A couple times I placed a dot, realized I actually didn't want it there and ran out of time to undo, which felt bad having to wait 30s. I wish it was a bit longer.
When you try to place a pixel a few milliseconds too early I feel like it should queue it and wait the few milliseconds for you.
I'm not super sure on the canvas having transparency. Most people treated the canvas as white, not transparent. If you wanted a white-on-white drawing, people will just make an outline.
Maybe a concept worth testing: if you place a pixel next to your own pixels, you get a (slightly) reduced cooldown, that way you get an extra boost when completing your art. (At the same time, I think there is beauty in the canvas being as simple as possible:)