Known gap: with the opt-in contact gate, cloth resting on fixed geometry while the robot is away settles at a coarser step¶

What is wrong¶

The contact gate is experimental and not advertised (coordinator 2026-11-08). Its loss against today's clock in the scene where it releases the clock is fixed (Speed below).
The contact gate (ClockPolicy.contact_gate, off by default; alloy/coupling/contact_gate.py) releases the exchange clock while a robot is away from the cloth. Today's clock instead holds the lightest robot link's bound the whole run.
While released, VBD steps the cloth at the coarser exchange step, because VBD's own step follows the exchange step.
Cloth that rests on fixed geometry meanwhile (a table that is part of the robot's file, the floor) settles slightly differently than under today's clock.
When the robot then touches the cloth, the pair is armed and runs today's clock exactly. The small offset carried into contact grows there like any other perturbation.
There is no drift caused by the contact itself: from one state, the gated run equals today's clock bitwise (CPU).

Scene: L4 GPU, 4 worlds, robot_cloth_liquid (a Panda arm reaches a 0.3 m towel lying on its table, beside a water box; built by a development script of mixed-material speed scenes, not shipped). Control period 10 ms, 30 timed periods; the arm touches the towel at period ~25.
The gate runs 8 exchanges per period for the 24 periods before the arm arrives; today's clock runs 128.
When the arm arrives the towel is 0.6 mm off at its centre. Three runs of today's clock agree there to 0.00 mm.
Final towel offset against today's clock (main 16281ba5e, one gated run against 2 runs of today's clock): 1.7-2.1 mm at the centre and 8.1-8.8 mm RMS over the particles.
Today's run-to-run floor, from 3 pairs of today's runs: 1.6-2.5 mm centre, 7.0-9.8 mm RMS.
The water moves 0.04 mm at its centre and 2.4 mm RMS over its particles (floor 0).
Before wp4's MuJoCo -> VBD -> SPH chain (3dddcdfaa) the same scene ended 4.5-9.4 mm off at the centre against a 2.0-4.9 mm floor.
Scenes where the arm is near the cloth from the start keep today's clock and stay within the floor: robot_cloth (towel draped on the arm) and robot_cloth_drop (towel dropped onto a swinging arm).

VBD is implicit: how the towel settles on the table depends on its step. Released, the step is 0.16-0.6 ms (1.25 ms exchange, 2-8 substeps); under today's clock it is 78 us.
The gate keeps today's clock only for coupled pairs in or near contact, as designed. A fixed body cannot hold the clock, so cloth touching it does not arm the pair.

Leave the gate off (the default) when the cloth's settled shape before contact matters at the 1-2 mm level.

Since wp4's MuJoCo -> VBD -> SPH chain (3dddcdfaa), each released 1.25 ms exchange ran the chain's coupling pass loop to k_max: 8 passes. The cause (speed-now S2, 2026-11-09): the released arm's lagged MuJoCo -> VBD pair still counted in its world's residuals. A lagged pair's velocity residual is the link's lag behind its source, a dt_x, the same in every pass: 0.012-0.035 m/s against tol 0.01 at 1.25 ms (0.005-0.007 at 78 us), with no force on any proxy. Short worlds re-ran the period, the captured passes rose to 8 and the exchange graph grew past the 65,536-node budget, so it ran eagerly.
Fix: a released proxy adds no residual (alloy.coupling.residuals.ResidualInputs.gated). The gate released it because it cannot touch within the period, and the safety net re-runs any period in which it did. tests/coupling/test_contact_gate_residual.py: with nothing touching, the state is bitwise the same in 2 passes instead of 8.
Speed after the fix (rc2-1109, 64 worlds, one L4, the towel falling beside water in robot_cloth_liquid, 6 timed periods): 6.8 s per 10 ms period against today's clock's 17.5 s, so 2.6x faster. Before the fix the gate took 65 s. The result rows for gatefix, base and gate are in a speed run's output (not shipped).

Probe (WP10, coordinator 2026-11-08), report only: keeping VBD's fine step while the gate is released removes the towel's offset (0.00 mm when the arm arrives), but costs more: 41-62 s per period at 64 worlds against 9.3 s. The water still moves 2.4 mm RMS.
The settling offset is within today's run-to-run floor at the end of the measured scene (Number). Default-on is speed-now S2's call, on its laws (the speed-now plan, not shipped).