fix(chrome): anchor the transition deadline when the dom goes quiet

it was anchored at script start, so a page that churned past the window
reached the check already expired and returned mid cross-fade. the
driver now publishes the idle timeout it needs, since the caller's 1s
could never spend the 800ms window.

internal/driver/chrome/driver.go

@@ -557,12 +557,35 @@ const domQuietPeriod = 150 * time.Millisecond
 // shows two *Screen ids at rest costs this much per step and no more.
 const transitionSettlePeriod = 800 * time.Millisecond
 
+// settleReturnMargin is what WaitForIdle holds back from the caller's timeout,
+// so returning late by our own doing surfaces as a settled page rather than a
+// context cancellation.
+const settleReturnMargin = 100 * time.Millisecond
+
+// settleScanMargin covers the in-page work the two waits do not themselves
+// account for: liveScreens() walks the document and every shadow root on each
+// 16 ms poll, and the whole script costs one CDP round trip.
+const settleScanMargin = 250 * time.Millisecond
+
+// MinIdleTimeout is the shortest timeout WaitForIdle can be handed and still
+// spend the waits it is built from: the DOM quiet period, the route-transition
+// window that only opens once that quiet period has elapsed, and the second
+// quiet period the transition's own closing mutation starts. A caller that
+// passes less caps the settle below its own budget, and the step then samples a
+// page that is still mid-transition - which is the exact failure the transition
+// wait exists to prevent. internal/runner raises a shorter caller timeout to
+// this value.
+func (d *Driver) MinIdleTimeout() time.Duration {
+	return 2*domQuietPeriod + transitionSettlePeriod +
+		settleScanMargin + settleReturnMargin
+}
+
 func (d *Driver) WaitForIdle(ctx context.Context, timeout time.Duration) error {
 	runCtx, cancel := d.runCtx(ctx)
 	defer cancel()
 	// Leave the caller's deadline some room: returning late by our own doing
 	// would surface as a context cancellation instead of a settled page.
-	budget := max(timeout-100*time.Millisecond, domQuietPeriod)
+	budget := max(timeout-settleReturnMargin, domQuietPeriod)
 	script := fmt.Sprintf(settleScript,
 		domQuietPeriod.Milliseconds(),
 		budget.Milliseconds(),
@@ -598,11 +621,18 @@ const liveScreensFunction = `
 // itself gives up after %d ms. Shadow roots get their own observer: a canvas
 // app keeps its whole accessibility tree inside one, and mutations there do not
 // reach an observer on the document.
+//
+// The transition window opens when the quiet period ends, not when the script
+// starts. Anchored at the start it is already spent by the time the check can
+// first run on any page that keeps mutating for longer than the window, so the
+// wait resolves immediately with both routes still live - the mid-transition
+// return this whole wait exists to prevent. Each mutation reopens it, and the
+// budget above bounds the total either way.
 const settleScript = `
 new Promise(resolve => {
   const quietMillis = %d, budgetMillis = %d, transitionMillis = %d;
   const observers = [];
-  const transitionDeadline = Date.now() + transitionMillis;
+  let transitionDeadline = 0;
   let timer = null;
   const finish = () => {
     clearTimeout(timer);
@@ -611,6 +641,7 @@ new Promise(resolve => {
   };
 ` + liveScreensFunction + `
   const quiet = () => {
+    if (transitionDeadline === 0) transitionDeadline = Date.now() + transitionMillis;
     if (liveScreens() > 1 && Date.now() < transitionDeadline) {
       timer = setTimeout(quiet, 16);
       return;
@@ -619,6 +650,7 @@ new Promise(resolve => {
   };
   const restart = () => {
     clearTimeout(timer);
+    transitionDeadline = 0;
     timer = setTimeout(quiet, quietMillis);
   };
   const watch = (root) => {
@@ -770,15 +802,19 @@ func (d *Driver) EvaluateExtractors(ctx context.Context) (map[int]json.RawMessag
 // action was actually applied. Without this call every web state.lastAction is
 // null, so a property gated on what the last action did is vacuously true and
 // reports a green run while checking nothing.
+//
+// The call is deliberately unguarded. A `setter && setter(...)` form evaluates
+// to undefined on a page whose runtime does not define the setter, and chromedp
+// reports that as success, so "the page cannot accept lastAction" would be
+// indistinguishable from "installed". That page is reachable: a run resolving
+// its web runtime from an older published @sanderling/spec would silently no-op
+// every step. Unguarded, the missing global throws and the run fails loudly.
 func (d *Driver) SetLastAction(ctx context.Context, encoded json.RawMessage) error {
 	payload := strings.TrimSpace(string(encoded))
 	if payload == "" {
 		payload = "null"
 	}
-	script := fmt.Sprintf(
-		`window.__sanderlingSetLastAction__ && window.__sanderlingSetLastAction__(%s)`,
-		payload,
-	)
+	script := fmt.Sprintf(`window.__sanderlingSetLastAction__(%s)`, payload)
 	runCtx, cancel := d.runCtx(ctx)
 	defer cancel()
 	if err := chromedp.Run(runCtx, chromedp.Evaluate(script, nil)); err != nil {
@@ -789,16 +825,25 @@ func (d *Driver) SetLastAction(ctx context.Context, encoded json.RawMessage) err
 
 // extractorScript resolves the extractor table once the page is not mid route
 // transition, giving up on that wait after %d ms.
+//
+// A missing table rejects rather than reporting {}, for the same reason
+// SetLastAction no longer guards its call: an empty override map is what a
+// spec with no extractors returns, so the guarded form made "this page has no
+// sanderling runtime" read as a normal step whose properties then ran on
+// goja's dump-derived values instead of the page's.
 const extractorScript = `
-new Promise(resolve => {
+new Promise((resolve, reject) => {
   const deadline = Date.now() + %d;` + liveScreensFunction + `
   const read = () => {
     if (liveScreens() > 1 && Date.now() < deadline) {
       setTimeout(read, 16);
       return;
     }
-    resolve(JSON.stringify(
-      window.__sanderlingExtractors__ ? window.__sanderlingExtractors__() : {}));
+    if (typeof window.__sanderlingExtractors__ !== "function") {
+      reject(new Error("__sanderlingExtractors__ is not installed in the page"));
+      return;
+    }
+    resolve(JSON.stringify(window.__sanderlingExtractors__()));
   };
   read();
 })`

internal/driver/chrome/driver_test.go

@@ -672,17 +672,31 @@ func TestWaitForIdle_ReturnsOnABusyPage(t *testing.T) {
 // app is leaving, which on the folio wasm build recorded a submit that had
 // landed on Home as still being on the transaction screen: the route gate of an
 // action-gated property then skipped the very step the action landed on.
+//
+// The page keeps mutating for 300 ms after the route splice, and the settle
+// runs on the timeout production hands it (MinIdleTimeout). Both details are
+// load-bearing. A quiet page reaches the transition check immediately, so it
+// passes whether the transition window is anchored at the script start or at
+// the end of the quiet period; churn is what pushes the check past a
+// start-anchored deadline, which then finishes at once with two live screens.
+// And a caller timeout below MinIdleTimeout cuts the whole settle off before
+// the transition window can be spent, which is the same bug from the other end.
 func TestWaitForIdle_WaitsOutARouteTransition(t *testing.T) {
 	const page = `<body><div id="app"></div><script>
 	  const root = document.getElementById("app").attachShadow({mode: "open"});
 	  root.innerHTML = '<button id="go" style="width:200px;height:80px">go</button>' +
-	    '<div id="LedgerScreen">ledger</div>';
+	    '<div id="LedgerScreen">ledger</div><div id="spinner">0</div>';
 	  root.getElementById("go").addEventListener("click", function () {
 	    const incoming = document.createElement("div");
 	    incoming.id = "HomeScreen";
 	    incoming.textContent = "home";
 	    root.appendChild(incoming);
-	    setTimeout(function () { root.getElementById("LedgerScreen").remove(); }, 400);
+	    let frame = 0;
+	    const churn = setInterval(function () {
+	      root.getElementById("spinner").textContent = String(++frame);
+	    }, 30);
+	    setTimeout(function () { clearInterval(churn); }, 300);
+	    setTimeout(function () { root.getElementById("LedgerScreen").remove(); }, 1000);
 	  });
 	</script></body>`
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
@@ -701,7 +715,7 @@ func TestWaitForIdle_WaitsOutARouteTransition(t *testing.T) {
 	if err := d.Tap(ctx, 40, 40); err != nil {
 		t.Fatalf("Tap: %v", err)
 	}
-	if err := d.WaitForIdle(ctx, 2*time.Second); err != nil {
+	if err := d.WaitForIdle(ctx, d.MinIdleTimeout()); err != nil {
 		t.Fatalf("WaitForIdle: %v", err)
 	}
 
@@ -793,3 +807,81 @@ func TestEvaluateExtractors_WaitsOutARouteTransition(t *testing.T) {
 			"mid-transition, so the spec sees the route the app is leaving", got)
 	}
 }
+
+// TestSetLastAction_ReportsAPageThatCannotTakeIt covers the install the whole
+// web path's action-gated properties hang off. A page without the setter is
+// reachable: internal/testrun resolves the web runtime from
+// node_modules/@sanderling/spec when no sibling checkout is present, and an
+// older published runtime does not define it. Guarded as
+// `setter && setter(...)`, that page returns undefined and chromedp reports
+// success, so every step silently no-ops and every property gated on the last
+// action goes vacuously true - a green run that checked nothing.
+func TestSetLastAction_ReportsAPageThatCannotTakeIt(t *testing.T) {
+	const withSetter = `<body><script>
+	  window.__lastActionSeen = null;
+	  window.__sanderlingSetLastAction__ = function (value) { window.__lastActionSeen = value; };
+	</script></body>`
+	const withoutSetter = `<body><div id="app">no sanderling runtime here</div></body>`
+	pages := map[string]string{"/with": withSetter, "/without": withoutSetter}
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "text/html")
+		_, _ = w.Write([]byte(pages[r.URL.Path]))
+	}))
+	defer server.Close()
+
+	d := New()
+	defer d.Terminate(context.Background())
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	if err := d.Launch(ctx, server.URL+"/with", false, nil); err != nil {
+		t.Fatalf("Launch: %v", err)
+	}
+	action := json.RawMessage(`{"kind":"Tap","on":"id:TxnSubmit"}`)
+	if err := d.SetLastAction(ctx, action); err != nil {
+		t.Fatalf("SetLastAction on a page that defines the setter: %v", err)
+	}
+	var seen map[string]string
+	if err := chromedp.Run(d.tabCtx,
+		chromedp.Evaluate(`window.__lastActionSeen`, &seen)); err != nil {
+		t.Fatalf("read installed action: %v", err)
+	}
+	if seen["on"] != "id:TxnSubmit" {
+		t.Errorf("the page received %v, want the action the runner applied", seen)
+	}
+
+	if err := d.Launch(ctx, server.URL+"/without", false, nil); err != nil {
+		t.Fatalf("Launch: %v", err)
+	}
+	if err := d.SetLastAction(ctx, action); err == nil {
+		t.Error("SetLastAction reported success on a page with no setter; " +
+			"a runtime that cannot take lastAction is indistinguishable from one that did")
+	}
+}
+
+// TestEvaluateExtractors_ReportsAMissingTable is the same failure on the other
+// sampler. An empty override map is what a spec with no extractors returns, so
+// treating a missing table as {} makes "this page has no sanderling runtime"
+// read as an ordinary step - and the verifier then judges the run on goja's
+// dump-derived values while believing they came from the page.
+func TestEvaluateExtractors_ReportsAMissingTable(t *testing.T) {
+	const page = `<body><div id="app">no sanderling runtime here</div></body>`
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "text/html")
+		_, _ = w.Write([]byte(page))
+	}))
+	defer server.Close()
+
+	d := New()
+	defer d.Terminate(context.Background())
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+	if err := d.Launch(ctx, server.URL, false, nil); err != nil {
+		t.Fatalf("Launch: %v", err)
+	}
+	values, err := d.EvaluateExtractors(ctx)
+	if err == nil {
+		t.Errorf("EvaluateExtractors returned %v and no error on a page with no "+
+			"extractor table; a page that cannot be read must not read as empty", values)
+	}
+}