fix(daemon): corroborate the no-effect claim against the rebased baseline

src/daemon/__tests__/post-gesture-stabilization.test.ts

@@ -158,6 +158,102 @@ test('capturePostGestureStabilizedResult keeps polling past the normal deadline
   assert.ok(captureCount > 8, `expected sustained polling, saw ${captureCount} captures`);
 });
 
+test('a backend flip mid-poll still yields the no-effect claim (#1620)', async () => {
+  // The screens this warning exists for are the hostile ones — #1600's
+  // element-18 was a Bluesky feed — and those are exactly the screens whose
+  // capture plan falls back mid-sequence. The loop already handles that: on a
+  // backend change it REBASES (#1569) and keeps polling against a comparable
+  // pair. The corroboration then read `pending.baselineSignature` instead, i.e.
+  // the pre-gesture signature from the OTHER backend, so set equality could
+  // never hold and the claim was vetoed on every fallback.
+  vi.useFakeTimers();
+  const session = makeSession('ios');
+  // Pre-gesture baseline captured by the TREE backend. The two backends do not
+  // agree on which nodes exist — that disagreement is the entire premise of
+  // #1569 — so private-ax additionally reports a scrolled-away row the tree
+  // backend prunes. Same screen, different view of it.
+  session.snapshot = makeSnapshotState(pickupSnapshot(500).nodes, {
+    snapshotQuality: { state: 'healthy', backend: 'tree' },
+  });
+  markPostGestureStabilization(session, 'scroll', ['up']);
+
+  // Every post-gesture capture comes from private-ax: the penalty armed during
+  // the gesture. They are byte-identical to EACH OTHER — the gesture genuinely
+  // moved nothing — while differing from the tree baseline by that extra row.
+  const privateAxNodes = [
+    ...pickupSnapshot(500).nodes,
+    {
+      index: 2,
+      parentIndex: 0,
+      type: 'StaticText',
+      identifier: 'scrolled-away-row',
+      label: 'Above the fold',
+      rect: { x: 20, y: -80, width: 200, height: 44 },
+    },
+  ];
+  const capture = vi.fn(async () =>
+    makeSnapshotState(privateAxNodes, {
+      snapshotQuality: { state: 'recovered', backend: 'private-ax' },
+    }),
+  );
+
+  const resultPromise = withDiagnosticsScope(
+    {},
+    async () =>
+      await capturePostGestureStabilizedResult({
+        session,
+        capture,
+        readSnapshot: (snapshot) => snapshot,
+      }),
+  );
+  await vi.advanceTimersByTimeAsync(10_000);
+  const result = await resultPromise;
+
+  assert.equal(
+    result.gestureNoEffect?.action,
+    'scroll',
+    'a proven-inert gesture must still be reported after the capture backend falls back',
+  );
+});
+
+test('no pre-gesture snapshot means no no-effect claim, even after a backend flip (#1622 P1)', async () => {
+  // `markPostGestureStabilization` records an EMPTY baseline when the session
+  // has no pre-gesture snapshot, and `[]` is truthy. Rebasing it on a backend
+  // flip would swap "no before-state" for a post-gesture capture, inventing the
+  // very evidence the claim is supposed to rest on — the loop would then agree
+  // with itself and report a gesture as inert with nothing to compare against.
+  vi.useFakeTimers();
+  const session = makeSession('ios');
+  session.snapshot = undefined; // nothing captured before the gesture
+  markPostGestureStabilization(session, 'scroll', ['up']);
+
+  // Steady private-AX captures: quiet, self-consistent, and a different backend
+  // from the (absent) baseline, so the rebase branch is reached.
+  const capture = vi.fn(async () =>
+    makeSnapshotState(pickupSnapshot(500).nodes, {
+      snapshotQuality: { state: 'recovered', backend: 'private-ax' },
+    }),
+  );
+
+  const resultPromise = withDiagnosticsScope(
+    {},
+    async () =>
+      await capturePostGestureStabilizedResult({
+        session,
+        capture,
+        readSnapshot: (snapshot) => snapshot,
+      }),
+  );
+  await vi.advanceTimersByTimeAsync(10_000);
+  const result = await resultPromise;
+
+  assert.equal(
+    result.gestureNoEffect,
+    undefined,
+    'a no-effect claim needs a real pre-gesture baseline, never one the loop invented for itself',
+  );
+});
+
 test('a replaced list under fixed chrome now settles outright, and still claims no no-effect (#1601 P1, #1569)', async () => {
   // The reviewer's counterexample: a SUCCESSFUL scroll swapped every list cell
   // while the tab-bar chrome (discriminating, shared, unmoved) kept the

src/daemon/post-gesture-stabilization.ts

@@ -214,7 +214,13 @@ export async function capturePostGestureStabilizedResult<T>(params: {
       // not agree on which nodes exist, so this pair says nothing about the
       // gesture: adopt it as the baseline and keep going rather than concluding
       // from it (#1569).
-      if (baselineSignature && baselineBackend !== current.backend) {
+      // `?.length`, not truthiness: `markPostGestureStabilization` records an
+      // EMPTY signature when there was no pre-gesture snapshot, and `[]` is
+      // truthy. Rebasing that would replace "no before-state" with a
+      // post-gesture capture — inventing a baseline the gesture is then judged
+      // against, which `decidePostGestureStabilityVerdict` (guarding on
+      // `?.length`) had deliberately refused to do (#1622 review P1).
+      if (baselineSignature?.length && baselineBackend !== current.backend) {
         emitDiagnostic({
           level: 'debug',
           phase: 'post_gesture_snapshot_baseline_rebased',
@@ -239,7 +245,7 @@ export async function capturePostGestureStabilizedResult<T>(params: {
       }
       clearPostGestureStabilization(session);
       emitPostGestureSettleDiagnostic(verdict, pending.action, attempts, elapsedMs);
-      return buildAcceptedStabilizedResult(verdict, pending, current);
+      return buildAcceptedStabilizedResult(verdict, pending, current, baselineSignature);
     }
     previous = current;
   }
@@ -280,15 +286,26 @@ export function formatGestureNoEffectWarning(action: string, positionals: string
  * corroboration (`haveIdenticalDiscriminatingSurfaces`): the verdict alone is
  * subset-tolerant, and a successful scroll that replaced every list cell
  * under fixed chrome still reads accept-stale (#1601 review P1).
+ *
+ * Corroborates against the loop's CURRENT baseline, not `pending`'s original
+ * one (#1620). When the capture backend flips mid-poll the loop rebases —
+ * `pending`'s pre-gesture signature came from a backend that does not agree
+ * with this one about which nodes exist, so #1569 already ruled it out for the
+ * verdict. Reading it back here re-introduced exactly that comparison, and set
+ * equality across two backends never holds: the corroboration was guaranteed to
+ * veto on any screen whose capture plan fell back — which is every hostile
+ * screen, i.e. the ones the warning exists for (#1600's element-18 was a
+ * Bluesky feed).
  */
 function buildAcceptedStabilizedResult<T>(
   verdict: 'trust' | 'accept-stale',
   pending: NonNullable<SessionState['postGestureStabilization']>,
   current: CapturedSurface<T>,
+  baselineSignature: InteractionSurfaceSignature | undefined,
 ): PostGestureStabilizedResult<T> {
   const corroborated =
     verdict === 'accept-stale' &&
-    haveIdenticalDiscriminatingSurfaces(pending.baselineSignature ?? [], current.signature);
+    haveIdenticalDiscriminatingSurfaces(baselineSignature ?? [], current.signature);
   if (!corroborated) return { value: current.value };
   return {
     value: current.value,