https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/paint/image_paint_timing_detector_test.cc?q=image_paint_timing+package:%5Echromium$&dr=C&l=127

When I was writing this test case, I discover that the layout object on the MacOS always has one extra pixel in width.

For example, when h*w = 5*6, the size result becomes 37; when h*x = 7*10, the size result becomes 77.

The size result in the test is not completely the layout size. It's the layout size that get transformed and intersected with viewport size. Extra work will be needed to tell whether the transform and intersecting have played a role in it.