I'm building an HTML page with a Mandelbrot set renderer that uses progressive rendering across frames to keep things responsive. I'll render to ImageData with a tiled approach, showing a low-resolution preview first while the full image fills in, and during panning or zooming I'll translate the existing canvas to give immediate visual feedback.

I'm verifying the skip logic for the coarse pass—at each step s, pixels where both coordinates are multiples of 2s get skipped since they were already computed at the coarser step 2s, which makes sense given how the coarse pass iterates. The initial pass at step 8 correctly has no skip condition, and since job.y increments by s, the y-coordinates are always multiples of s as expected. Now moving on to the progress bar.

The scaling formula is a bit rough but shouldn't cause issues. I'm checking the resize logic—there's a potential noop with `scale = scale`, though the fallback `if(!oldW) scale = 3.2/W` handles the edge case correctly. For periodicity detection, initializing pr and pi to 0 could theoretically flag z=0 as exterior, but after the first iteration with z=0 and some c, we get zr=cr and zi=ci which are nonzero unless c itself is zero, and that's interior anyway, so it's fine.