About the fill tool request:

Theres a popular drawing game website, you must know: drawception.com
Just like anondraw, it have only simple drawing tools (no fill bucket). So, the user https://drawception.com/player/342373/grom-pe/ made a script (https://github.com/grompe/Drawception-ANBT) wich runs as a Tampermonkey extension that have some features... including a fill tool. I guess you could take a look there: maybe the code can be useful as a reference to you implement the feature(s) here aswell.