In case flex items had `margin: auto` on the primary flex axis, we were
still also distributing remaining space according to `justify-content`
rules. This lead to duplicated spacing in various places and overflows.
It looks like this issue was observed previously but missidentified
because there was logic to ignore margins at the start and end which
would partially paper over the root cause. However this created other
bugs (like for example not having a margin at beginning and end ;-)) and
I can find nothing in the spec or other browser behaviour that indicates
that this is something that should be done.
Now we skip justify-content space distribution alltogether if it has
already been distributed to auto margins.
