When penetration exceeds a box's smallest half-size, the midpoint
between the contact surfaces can land outside both boxes. The outside-
box filter in mjc_BoxBox then removed every contact of the manifold,
returning nothing for a visibly overlapping pair, letting boxes fall
through each other. Fixes#1800.
If the filter would remove all contacts, restore the penetrating ones.
This is strictly additive: configurations where any contact survives
the filter are unchanged.
The removed midpoints are bitwise-identical to the witness midpoints
computed by mj_geomDistance for these configurations, so the positions
follow the engine-wide contact position convention; re-anchoring them
onto a box surface would not.
Fixes#1800
PiperOrigin-RevId: 957867315
Change-Id: Ia9c858661d4badeb2a832d25455e33402936011d
Yuval Tassa
fb07a9ca50