Sorry, I misremembered how this works for grid -- this is indeed a flex-specific thing, I think. No change needed for grid. --> Editing description and comment 0. (In particular, the bit that I was forgetting was: align-content and justify-content don't directly influence the position of grid items or abspos descendants; they only set the position of the grid areas, which then serve as the containing blocks for those items/descendants. In contrast: as of bug 1269046, flex containers' align/justify-content properties were supposed to directly influence the alignment of abspos flex children (e.g. centering or end-aligning them within the flex container itself), and apparently the spec has changed so that now they do not.)
Bug 1682641 Comment 3 Edit History
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Sorry, I misremembered how this works for grid -- this is indeed a flex-specific thing, I think. No change needed for grid. --> Editing description and comment 0. (In particular, the bit that I was forgetting was: align-content and justify-content don't directly influence the position of grid items or a grid's abspos descendants; they only set the position of the grid areas, which then serve as the containing blocks for those items/descendants. In contrast: as of bug 1269046, a flex container's align/justify-content properties were supposed to directly influence the alignment of abspos flex children (e.g. centering or end-aligning them within the flex container itself), and apparently the spec has changed so that now they do not.)