Good catch! I think this is a valid bug. We're probably calculating the flex container's hypothetical size incorrectly here: https://searchfox.org/mozilla-central/source/layout/generic/nsFlexContainerFrame.cpp#5216-5236 We're adding n-1 gaps (where n=number of in-flow child frames), and we need to exclude the 0-sized nsPlaceHolderFrame children when we do that.
Bug 1612401 Comment 1 Edit History
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Good catch! I think this is a valid bug. We're probably calculating the flex container's hypothetical size incorrectly here: https://searchfox.org/mozilla-central/rev/2e355fa82aaa87e8424a9927c8136be184eeb6c7/layout/generic/nsFlexContainerFrame.cpp#5216-5236 We're adding n-1 gaps (where n=number of in-flow child frames), and we need to exclude the 0-sized nsPlaceHolderFrame children when we do that.