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.
The draft CSS-FLEXBOX-1 spec had a more detailed description of this
algorithm, so let's use that as our basis for the implementation.
Test by Aliaksandr. :^)
When resolving these constraints to CSS pixel sizes, we have to resolve
padding-top and padding-bottom against the flex container's *width*,
not its height.
The padding-top and padding-bottom properties are relative to the
*width* of the containing block, not the height.
It's funny how we keep making this same mistake again and again. :^)
If a box has clearance and margin bottom of preceding box is greater
than static y of the box then it should also affect y offset in current
block container so subsequent boxes will get correct y position too.
Though table wrappers are anonymous block containers (because
TableWrapper is inherited from BlockContainer) with no lines they
should not be skipped in block auto height calculation.
