I've tried writing a fast flood fill in JS and it's a lot more challenging than you and GP make it out to be. If it's really so straightforward, you should produce code. Otherwise, these comments don't add much to the discussion.

i'd be surprised if someone could do it within the editing window for an hn comment, but it doesn't seem like more than a day or two of work for double-digit megapixel fill rates

I wasn’t calling per pixel :-) even when just visiting each pixel once it would still take 50 - 100ms to fill a large image which the user perceives as lag. I wanted it to be instant