wplace-fill-region.js

let color = 0;
const mins = {x: Infinity, y: Infinity};
const maxs = {x: -Infinity, y: -Infinity};

for( let tile of n.tiles ){
    for( let x of tile.pixels.x ) {
        mins.x = Math.min(mins.x, tile.x * 1000 + x);
        maxs.x = Math.max(maxs.x, tile.x * 1000 + x);
    }
    for( let y of tile.pixels.y ) {
        mins.y = Math.min(mins.y, tile.y * 1000 + y);
        maxs.y = Math.max(maxs.y, tile.y * 1000 + y);
    }
    color = tile.pixels.colors[0];
}
maxs.x += 1;
maxs.y += 1;



const coords = [];
for(let x = mins.x; x < maxs.x; x++){
    for(let y = mins.y; y < maxs.y; y++){
        coords.push([x, y])
    }
}
console.log(coords);

const tiles = new Map();
for( let [x, y] of coords ){
    const tx = Math.floor(x * 0.001);
    const ty = Math.floor(y * 0.001);
    let tile;
    if( !tiles.has(`${tx},${ty}`) ){
        tile = {
            x: tx,
            y: ty,
            pixels: {
                x: [],
                y: [],
                colors: []
            }
        };
        tiles.set(`${tx},${ty}`, tile);
    }
    tile ??= tiles.get(`${tx},${ty}`);

    tile.pixels.x.push(x % 1000);
    tile.pixels.y.push(y % 1000);
    tile.pixels.colors.push(color);
}

n.tiles = [...tiles.values()];