Root cause (measured, not guessed): the OOM was a MANAGED Large-Object-Heap leak
(~5 GB in 6 min of roaming; working set 1.6 GB -> 7.6 GB), dominated by ~3.8 GB of
live System.Single[]. A heap-retention analysis of a captured gcdump traced 241 of
242 giant float arrays to a single List<float>: WbDrawDispatcher.InstanceGroup.Opacities.
#188 (2026-07-09, fading doors) added Opacities as a 5th per-instance parallel list
alongside Matrices/Slots/LightSets/IndoorFlags, but left it out of the per-frame
clear loop (WbDrawDispatcher.cs:959). So Opacities.Add(1.0f) fired once per drawn
instance per frame and the list never reset — growing forever; List<float> capacity
doubling produced the observed ~128 MB / ~512 MB power-of-two arrays and OOM after
~50 min. Hit both UI builds because it's core render, and started only recently
because Opacities did not exist before #188.
Fix: extract the per-frame reset into InstanceGroup.ClearPerInstanceData() (promoted
InstanceGroup private->internal) that clears ALL FIVE parallel lists in one place, so
a future 6th list can't silently drift out of the frame lifecycle again. Line 959 now
calls it. TDD: InstanceGroupClearTests asserts every parallel list resets (RED with
the old 4-list clear, GREEN after adding Opacities.Clear()).
Static-audit note: this is exactly why we measured before fixing — the audit ranked
GlobalMeshBuffer (GPU) #1 and the entity-dict leak #3; both were wrong about the
dominant cause. The dotnet-counters (managed vs native) + gcdump retention graph
named the real one-line culprit.
Secondary (NOT this commit): _groups dictionary is never pruned (bounded by distinct
group keys, empty groups are cheap) — filed as a follow-up, not the multi-GB driver.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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