Solve the “items on the last line get way too big when you're flexing” problem. More generally, “make items have a consistent flexed size, regardless of how much extra space is on each line”.
Possible solution - fill out the last line with “phantom copies” of the last item, flex with them in, then remove them.
Possible solution - calculate minimum values of 1fr and alignment free space across the entire flexbox (instead of per-line) and use that.
Flex line balancing.
Tight packing - if items don't fit on one line, try to pack them again with their min-content size instead, causing line wraps inside the item before wrapping the flex line (good for toolbars).
Explicit spacing control - specify the size/flex of a particular packing space. (Idea - copy Grid and use flex-main-gap/flex-cross-gap properties for general control over min gap.)
Equal justification space in both dimensions (see mailing list). (Or maybe just explicit sizing of between-line spacing?)
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Some way to specify that things should “collapse” (like box-suppress:hide, or discard?) rather than “shrink” when there's not enough space. Need a way to specify collapsing priority, too.
