From b870f446c69b9a2f7ec4d1e4316734f8d679dca4 Mon Sep 17 00:00:00 2001
From: German Khokhlov <ouchlock@gmail.com>
Date: Wed, 11 Nov 2026 02:15:52 +0400
Subject: [PATCH] Let the cold-start comparison excuse a field within a bound

Field entries in the ledger allowlist now apply to every difference row,
in the ledger and in the probe alike, and may carry maxDelta. The traffic
fill gets one with a bound of three: its simulated vehicles enter and
leave every frame, so two pages snapshotted moments apart differ by a
vehicle or two.
---
 scripts/qa-ledger-allow.json |  8 ++++--
 scripts/qa/coldStart.mjs     | 52 +++++++++++++++++++-----------------
 2 files changed, 34 insertions(+), 26 deletions(-)

diff --git a/scripts/qa-ledger-allow.json b/scripts/qa-ledger-allow.json
index fc7ed8d27..621d596c7 100644
--- a/scripts/qa-ledger-allow.json
+++ b/scripts/qa-ledger-allow.json
@@ -1,14 +1,18 @@
 [
   {
     "id": "local-dams-footprints",
-    "family": "ground",
     "field": "count",
     "reason": "Infrastructure level of detail keeps features chosen on the way to a view (incumbents and eviction grace, so footprints do not flicker while panning); a cold start reaches the view without that path."
   },
   {
     "id": "local-datacenters-footprints",
-    "family": "ground",
     "field": "count",
     "reason": "Infrastructure level of detail keeps features chosen on the way to a view (incumbents and eviction grace, so footprints do not flicker while panning); a cold start reaches the view without that path."
+  },
+  {
+    "id": "traffic-fill",
+    "field": "count",
+    "maxDelta": 3,
+    "reason": "Simulated vehicles enter and leave the traffic fill every frame, so two pages snapshotted at slightly different moments differ by a vehicle or two."
   }
 ]
diff --git a/scripts/qa/coldStart.mjs b/scripts/qa/coldStart.mjs
index fdd6b39f7..47efedcf3 100644
--- a/scripts/qa/coldStart.mjs
+++ b/scripts/qa/coldStart.mjs
@@ -317,30 +317,16 @@ export function compareColdStart(
     const next = cold.ledger.find(
       (other) => other.family === row.family && other.id === row.id,
     );
-    if (next) {
-      // A field entry excuses one field of one set, with its reason.
-      const rows = compareProbe(
-        { kind: row.kind, count: row.count },
-        { kind: next.kind, count: next.count },
-      ).filter((difference) => {
-        const exception = allow.find(
-          (entry) =>
-            entry.field === difference.field &&
-            entry.id === row.id &&
-            (!entry.family || entry.family === row.family),
-        );
-        if (exception)
-          skipped.push({
-            layer: layerOf(row.family, row.id),
-            family: row.family,
-            id: row.id,
-            ...difference,
-            reason: exception.reason,
-          });
-        return !exception;
-      });
-      add(layerOf(row.family, row.id), row.family, row.id, rows);
-    }
+    if (next)
+      add(
+        layerOf(row.family, row.id),
+        row.family,
+        row.id,
+        compareProbe(
+          { kind: row.kind, count: row.count },
+          { kind: next.kind, count: next.count },
+        ),
+      );
   }
   for (const layer of new Set([
     ...Object.keys(switched.layers),
@@ -369,8 +355,26 @@ export function compareColdStart(
       id,
       compareProbe(a.get(id), b.get(id)),
     );
+  // A field entry excuses one field of one set (in the ledger and the probe),
+  // within maxDelta when it has one, with its reason.
+  const excused = (row) =>
+    allow.find(
+      (entry) =>
+        entry.field === row.field &&
+        entry.id === row.id &&
+        (!entry.family ||
+          entry.family === row.family.replace(/^probe-/, '')) &&
+        (entry.maxDelta === undefined ||
+          Math.abs(Number(row.switchValue) - Number(row.coldStartValue)) <=
+            entry.maxDelta),
+    );
   return {
     differences: differences.filter((row) => {
+      const exception = excused(row);
+      if (exception) {
+        skipped.push({ ...row, reason: exception.reason });
+        return false;
+      }
       const field = sessionState && sessionExclusion(row, switched);
       if (!field) return true;
       skipped.push({ ...row, sessionState: field });