I dunno, move the little circles around I guess. Sorry about how if you move your mouse to fast it will let go of the points, I was too lazy to fix that. Press space to test the Tris Per Second (TPS) - I don't know whether this is fast on the scale of tri fillers, but I can't really see a way to optimise it, since it's just math. I get ~1254 TPS on my computer, and ~614 TPS on my dad's cumbly old laptop. This doesn't work very well with transparent pen.
After learning cool math stuff I decided to come back to scratch and post this :). Feel free to use this, I don't really care. If you want to use this, just backpack the block that is named "fill tri" If you are wondering about the math behind it... go look it up. I'm not explaining. Ok fine. But only a little. Step 1. First, it calculates the incenter of the triangle, then the radius of the incircle. The incircle is a circle that is located within a triangle in a position that will allow it to have the highest possible radius without passing out of the triangle bounds. Step 2. Then it points towards a vertex of the triangle, and moves towards it, changing the pen size down each step by a specified amount so that everything is within triangle bounds. It repeats step 2 for the other two vertices of the triangle. Tada! Done! If you want to know how it calculates the incenter and the radius of the incircle, either look inside the project, or get ChatGPT to explain it to you and follow along in the Desmos Geometry Calculator. Have a great day!
