I'm moving this from issue #808758 (I'll leave that issue to investigate
the grid related problem reported originally there).

This is a generic problem happening with orthogonal elements and percentage paddings.

Check the attached example, it was working fine before:
https://chromium.googlesource.com/chromium/src/+/3d7b19bbee5aac29c8efa9df62f08264c478041b

So it's somehow a regression caused by that patch.

See that when you have "X X X X X" in the content it works fine ("Good"),
but when you change it to "X X X X X X" it starts to behave badly ("Bad").
Note that the padding-top is properly calculated as 50px in both cases,
but the "Bad" one paints the content in the parent too (which is quite strange).

Note that the very same case in Grid Layout works fine (2nd example attached),
in grid we ignore grid items in LocalFrameView::LayoutOrthogonalWritingModeRoots()
(see PrepareOrthogonalWritingModeRootForLayout()).

 

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