* 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?)
  * Some way to control how many lines the flexbox is allowed to break over (so items will start shrinking if there's still not enough space) [[http://lists.w3.org/Archives/Public/www-style/2014Sep/0230.html]]
  * 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.