This is a debug assertion checking multicol containing block is equal to or is an ancestor of {ib} containing block.
When climbing the parent chain, `GetIBContainingBlockFor()` skips any frame with pseudo or anon style in https://searchfox.org/mozilla-central/rev/9ee63566281365f26e7a4b06c9d4e2219e64c3e8/layout/base/nsCSSFrameConstructor.cpp#558
In this test case, the multicol containing block is a `ColumnSetWrapper(body)` with `-moz-scrolled-content` style. Therefore `GetIBContainingBlockFor()` just skips it and reach the `HTMLScroll(body)` above the multicol containing block.
The logic to skipping pseudo or anon frames was added in Bug 322678. We need to figure out whether it's still needed, or we should adjust the assertion.
Bug 1541679 Comment 1 Edit History
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This is a debug assertion checking multicol containing block is equal to or is an ancestor of {ib} containing block.
When climbing the parent chain, `GetIBContainingBlockFor()` skips any frames with pseudo or anon style in https://searchfox.org/mozilla-central/rev/9ee63566281365f26e7a4b06c9d4e2219e64c3e8/layout/base/nsCSSFrameConstructor.cpp#558
In this test case, the multicol containing block is a `ColumnSetWrapper(body)` with `-moz-scrolled-content` style. Therefore `GetIBContainingBlockFor()` just skips it and reach the `HTMLScroll(body)` above the multicol containing block.
The logic to skipping pseudo or anon frames was added in Bug 322678. We need to figure out whether it's still needed, or we should adjust the assertion.