refactor(hierarchy): one Tree.Transitional, drop the duplicated cross-fade check

internal/hierarchy/hierarchy.go

@@ -272,6 +272,25 @@ func elementFromNode(node *treeNodeJSON) *Element {
 	return element
 }
 
+// Transitional reports more than one resource id ending in "Screen": the marker
+// of a Compose NavHost mid cross-fade, where the source and destination route
+// composables are both alive in a collapsed, mid-animation layout.
+func (t *Tree) Transitional() bool {
+	if t == nil {
+		return false
+	}
+	screens := 0
+	for _, element := range t.Elements {
+		if strings.HasSuffix(element.ResourceID, "Screen") {
+			screens++
+			if screens > 1 {
+				return true
+			}
+		}
+	}
+	return false
+}
+
 // Find returns the first element matching the selector, or nil.
 func (t *Tree) Find(selector string) *Element {
 	node := t.FindNode(selector)

internal/hierarchy/hierarchy_test.go

@@ -1006,3 +1006,32 @@ func TestParseBoundsRejectsBadInput(t *testing.T) {
 		}
 	}
 }
+
+// Bug class: a NavHost cross-fade carries two route-level *Screen ids at once;
+// both the runner's re-fetch guard and the LLM's candidate enumeration depend on
+// spotting it, and neither must flag a settled single-screen tree.
+func TestTreeTransitional(t *testing.T) {
+	multi, err := Parse(`{"attributes":{"resource-id":"root"},"children":[
+	  {"attributes":{"resource-id":"AddAccountScreen"},"children":[]},
+	  {"attributes":{"resource-id":"HomeScreen"},"children":[]}
+	]}`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !multi.Transitional() {
+		t.Error("expected multi-screen tree to be flagged as transitional")
+	}
+
+	single, err := Parse(`{"attributes":{"resource-id":"HomeScreen"},"children":[]}`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if single.Transitional() {
+		t.Error("single-screen tree must not be flagged as transitional")
+	}
+
+	var nilTree *Tree
+	if nilTree.Transitional() {
+		t.Error("nil tree must not be flagged as transitional")
+	}
+}

internal/runner/runner.go

@@ -878,7 +878,7 @@ retryLoop:
 			tree, err = hierarchy.Parse(hierarchyJSON)
 			pngBytes = image.PNG
 		}
-		if err != nil || !isTransitionalHierarchy(tree) {
+		if err != nil || !tree.Transitional() {
 			break
 		}
 		// A tree unchanged since the previous attempt is a settled state
@@ -909,27 +909,6 @@ retryLoop:
 	return tree, pngBytes, transitional, err
 }
 
-// isTransitionalHierarchy returns true when the tree carries more than one
-// resource-id ending in "Screen" - the marker of a Compose NavHost mid
-// cross-fade where both source and destination route composables are alive.
-// Mirrors the sidecar's stabilitySnapshot heuristic so runner-side rejection
-// stays consistent with the settle poll.
-func isTransitionalHierarchy(tree *hierarchy.Tree) bool {
-	if tree == nil {
-		return false
-	}
-	screens := 0
-	for _, element := range tree.Elements {
-		if strings.HasSuffix(element.ResourceID, "Screen") {
-			screens++
-			if screens > 1 {
-				return true
-			}
-		}
-	}
-	return false
-}
-
 func traceActionFor(action verifier.Action, tree *hierarchy.Tree) *trace.Action {
 	traceAction := &trace.Action{Kind: string(action.Kind), X: action.X, Y: action.Y}
 	switch action.Kind {

internal/runner/runner_test.go

@@ -1118,34 +1118,6 @@ func TestRunner_OneScreenshotPerStep(t *testing.T) {
 	}
 }
 
-// TestIsTransitionalHierarchy_DetectsMultipleScreens covers the runner-side
-// guard that re-fetches when the hierarchy still carries two route-level
-// *Screen ids - the NavHost cross-fade signature.
-func TestIsTransitionalHierarchy_DetectsMultipleScreens(t *testing.T) {
-	multi, err := hierarchy.Parse(`{"attributes":{"resource-id":"root"},"children":[
-	  {"attributes":{"resource-id":"AddAccountScreen"},"children":[]},
-	  {"attributes":{"resource-id":"HomeScreen"},"children":[]}
-	]}`)
-	if err != nil {
-		t.Fatal(err)
-	}
-	if !isTransitionalHierarchy(multi) {
-		t.Error("expected multi-screen tree to be flagged as transitional")
-	}
-
-	single, err := hierarchy.Parse(`{"attributes":{"resource-id":"HomeScreen"},"children":[]}`)
-	if err != nil {
-		t.Fatal(err)
-	}
-	if isTransitionalHierarchy(single) {
-		t.Error("single-screen tree must not be flagged as transitional")
-	}
-
-	if isTransitionalHierarchy(nil) {
-		t.Error("nil tree must not be flagged as transitional")
-	}
-}
-
 // TestRunner_StableTransitionalTreeIsVerified feeds a driver whose hierarchy
 // constantly carries two route-level *Screen ids but never changes between
 // retry attempts. Such a tree is a settled state that merely matches the

internal/verifier/llm.go

@@ -130,22 +130,6 @@ type ActionCandidate struct {
 	prob float64
 }
 
-// transitionalTree reports a NavHost cross-fade: more than one route *Screen
-// alive at once. Mirrors runner.isTransitionalHierarchy so the LLM rejects the
-// same frames the verifier's transitional handling does.
-func transitionalTree(tree *hierarchy.Tree) bool {
-	screens := 0
-	for _, element := range tree.Elements {
-		if strings.HasSuffix(element.ResourceID, "Screen") {
-			screens++
-			if screens > 1 {
-				return true
-			}
-		}
-	}
-	return false
-}
-
 // verbActionKind maps a picker verb to the action kind it dispatches.
 func verbActionKind(verb string) ActionKind {
 	switch verb {
@@ -188,12 +172,10 @@ func (v *Verifier) Candidates() []ActionCandidate {
 	if v.lastTree == nil {
 		return nil
 	}
-	// A cross-fade frame carries more than one route *Screen at once (a NavHost
-	// mid-transition); its layout is mid-animation, often in a collapsed
+	// A cross-fade frame's layout is mid-animation, often in a collapsed
 	// coordinate space, so acting on it taps garbage (e.g. the soft keyboard).
-	// Skip it so the LLM re-observes a settled frame next step. Mirrors the
-	// runner's isTransitionalHierarchy.
-	if transitionalTree(v.lastTree) {
+	// Skip it so the LLM re-observes a settled frame next step.
+	if v.lastTree.Transitional() {
 		return nil
 	}
 	root := v.runtime.GlobalObject().Get("actions")