Teach it to stop, not just to click

Agentic computer-use RL is reported in single runs, and those numbers mislead. Using verifier-guided repair of a 35B computer-use agent (CUA) across five oracle-graded environments, we show a repaired policy's success rate is dominated by upstream variance: a variance-components decomposition across three cells (crossed data-draw × seed grid, bootstrap CIs) finds evaluation variance negligible (σeval ≈ 0) and the training-seed effect small everywhere (≤ 10%); instead it splits between the data draw and run-to-run nondeterminism, the data draw's share rising to dominant (48%) on the hardest cell. There the run-to-run distribution is bimodal (Hartigan dip p=0.07, k=10), so a single run has roughly a 30% chance of the failure mode and mean±std is the wrong summary. On that footing, two findings hold. First, repairability is two-tier in how constrained the corrective action is: a single fixed token installs reliably (done-detection 0.97±0.06), while open-ended corrections are only partial -- spatial-coordinate clicks (grounding 0.53±0.35) and a generative field-fill (0.14±0.04). Second, the frame-level repair transfers to task success only when the corrective action is the task's sole remaining blocker (LinkedIn 8/20 vs. base 0/15, Fisher p=0.006). We caught two of our own over-claims -- a sample-efficiency curve and a 'grounding cannot be bought' boundary -- only by replicating across seeds; a stress test makes the stakes external: a single-run improvement of the size this field publishes would have the wrong sign roughly one-third of the time in a comparable regime. We release a library (cuareliability) for routine k-seed reporting. The apparatus is, to our knowledge, the first multimodal segment-aggregated on-policy self-distillation (SA-OPSD) update on a real 35B CUA policy.