refactor(verifier): derive the lastAction shape from one field list

both hosts must show a spec the same lastAction. one ordered list now
feeds the goja object and the json the web host installs, so they
cannot drift.

internal/verifier/marshal.go

@@ -1,6 +1,7 @@
 package verifier
 
 import (
+	"bytes"
 	"encoding/json"
 	"fmt"
 	"strings"
@@ -325,49 +326,118 @@ func selectorObjectToString(runtime *goja.Runtime, arg goja.Value) string {
 	return strings.Join(parts, " ")
 }
 
+// actionField is one property of the lastAction object, in the order the
+// object is built. Value is a string, an int, or a nested []actionField for
+// the from/to points.
+type actionField struct {
+	key   string
+	value any
+}
+
+// lastActionFields is the ONE description of the lastAction shape. The goja
+// host turns it into a JS object (lastActionObject); the web host receives the
+// same fields as JSON (EncodeLastAction) and installs them as state.lastAction
+// in the page. Both hosts therefore expose identical field names, casing,
+// presence and order, so a property reading state.lastAction cannot mean one
+// thing on native and another on web.
+func lastActionFields(action *Action) []actionField {
+	point := func(x, y int) []actionField {
+		return []actionField{{key: "x", value: x}, {key: "y", value: y}}
+	}
+	fields := []actionField{{key: "kind", value: string(action.Kind)}}
+	if action.On != "" {
+		fields = append(fields, actionField{key: "on", value: action.On})
+	}
+	if action.Text != "" {
+		fields = append(fields, actionField{key: "text", value: action.Text})
+	}
+	switch action.Kind {
+	case ActionKindSwipe:
+		fields = append(fields,
+			actionField{key: "from", value: point(action.FromX, action.FromY)},
+			actionField{key: "to", value: point(action.ToX, action.ToY)})
+		if action.DurationMillis > 0 {
+			fields = append(fields,
+				actionField{key: "durationMillis", value: action.DurationMillis})
+		}
+	case ActionKindScroll:
+		fields = append(fields,
+			actionField{key: "direction", value: action.Direction},
+			actionField{key: "from", value: point(action.FromX, action.FromY)},
+			actionField{key: "to", value: point(action.ToX, action.ToY)})
+	case ActionKindPressKey:
+		fields = append(fields, actionField{key: "key", value: action.Key})
+	case ActionKindWait:
+		fields = append(fields,
+			actionField{key: "durationMillis", value: action.DurationMillis})
+	}
+	return fields
+}
+
 func lastActionObject(runtime *goja.Runtime, action *Action) goja.Value {
 	if action == nil {
 		return goja.Null()
 	}
+	return objectFromFields(runtime, lastActionFields(action))
+}
+
+func objectFromFields(runtime *goja.Runtime, fields []actionField) *goja.Object {
 	object := runtime.NewObject()
-	_ = object.Set("kind", string(action.Kind))
-	if action.On != "" {
-		_ = object.Set("on", action.On)
+	for _, field := range fields {
+		if nested, ok := field.value.([]actionField); ok {
+			_ = object.Set(field.key, objectFromFields(runtime, nested))
+			continue
 		}
-	if action.Text != "" {
-		_ = object.Set("text", action.Text)
-	}
-	switch action.Kind {
-	case ActionKindSwipe:
-		from := runtime.NewObject()
-		_ = from.Set("x", action.FromX)
-		_ = from.Set("y", action.FromY)
-		to := runtime.NewObject()
-		_ = to.Set("x", action.ToX)
-		_ = to.Set("y", action.ToY)
-		_ = object.Set("from", from)
-		_ = object.Set("to", to)
-		if action.DurationMillis > 0 {
-			_ = object.Set("durationMillis", action.DurationMillis)
-		}
-	case ActionKindScroll:
-		_ = object.Set("direction", action.Direction)
-		from := runtime.NewObject()
-		_ = from.Set("x", action.FromX)
-		_ = from.Set("y", action.FromY)
-		to := runtime.NewObject()
-		_ = to.Set("x", action.ToX)
-		_ = to.Set("y", action.ToY)
-		_ = object.Set("from", from)
-		_ = object.Set("to", to)
-	case ActionKindPressKey:
-		_ = object.Set("key", action.Key)
-	case ActionKindWait:
-		_ = object.Set("durationMillis", action.DurationMillis)
+		_ = object.Set(field.key, field.value)
 	}
 	return object
 }
 
+// EncodeLastAction renders the previous step's action for the web host, which
+// has no Go-side state object to read: the runner pushes this JSON into the
+// page before each extractor evaluation. A nil action encodes as JSON null,
+// the same value the goja host reports on the first step of a run and after a
+// step whose action was never applied.
+func EncodeLastAction(action *Action) json.RawMessage {
+	if action == nil {
+		return json.RawMessage("null")
+	}
+	return encodeFields(lastActionFields(action))
+}
+
+func encodeFields(fields []actionField) json.RawMessage {
+	var buffer bytes.Buffer
+	buffer.WriteByte('{')
+	for index, field := range fields {
+		if index > 0 {
+			buffer.WriteByte(',')
+		}
+		buffer.Write(encodeJSValue(field.key))
+		buffer.WriteByte(':')
+		if nested, ok := field.value.([]actionField); ok {
+			buffer.Write(encodeFields(nested))
+			continue
+		}
+		buffer.Write(encodeJSValue(field.value))
+	}
+	buffer.WriteByte('}')
+	return buffer.Bytes()
+}
+
+// encodeJSValue encodes one value the way JS JSON.stringify would, so the JSON
+// the web host parses is byte-identical to what the goja object stringifies to.
+// Go escapes <, > and & by default, which JSON.stringify does not, and that
+// alone would make the two hosts encode the same selector differently.
+func encodeJSValue(value any) []byte {
+	var buffer bytes.Buffer
+	encoder := json.NewEncoder(&buffer)
+	encoder.SetEscapeHTML(false)
+	if err := encoder.Encode(value); err != nil {
+		return []byte("null")
+	}
+	return bytes.TrimRight(buffer.Bytes(), "\n")
+}
+
 func runtimeMillis(stepTime, runStart time.Time) int64 {
 	if stepTime.IsZero() || runStart.IsZero() {
 		return 0

internal/verifier/marshal_test.go

@@ -147,3 +147,49 @@ func TestLastActionObject_ExposesKindSpecificFields(t *testing.T) {
 		}
 	})
 }
+
+// TestLastAction_WebJSONMatchesTheGojaObject pins the two hosts to ONE shape.
+// The goja host builds state.lastAction as a JS object; the web host receives
+// EncodeLastAction's JSON and installs the parsed value as state.lastAction in
+// the page. A field this side renames, drops or cases differently would leave a
+// spec reading state.lastAction working on native and silently mismatching on
+// web, which is the failure this whole path exists to prevent. Comparing
+// goja's own JSON.stringify against the encoder is the strongest available
+// statement that the two are the same object.
+func TestLastAction_WebJSONMatchesTheGojaObject(t *testing.T) {
+	verifier := newVerifier(t)
+	mustLoad(t, verifier, `
+		globalThis.last = __sanderling__.extract(state => JSON.stringify(state.lastAction));
+	`)
+
+	for _, testCase := range []struct {
+		name   string
+		action *Action
+	}{
+		{"nil", nil},
+		{"Tap", &Action{Kind: ActionKindTap, On: "id:TxnSubmit", X: 12, Y: 34}},
+		{"TapWithoutSelector", &Action{Kind: ActionKindTap, X: 12, Y: 34}},
+		{"DoubleTap", &Action{Kind: ActionKindDoubleTap, On: `desc:say "hi" <b>`}},
+		{"InputText", &Action{Kind: ActionKindInputText, On: "id:field", Text: "50"}},
+		{"Swipe", &Action{Kind: ActionKindSwipe, FromX: 1, FromY: 2, ToX: 3, ToY: 4, DurationMillis: 250}},
+		{"SwipeNoDuration", &Action{Kind: ActionKindSwipe, FromX: 1, FromY: 2, ToX: 3, ToY: 4}},
+		{"Scroll", &Action{Kind: ActionKindScroll, Direction: "down", FromX: 5, FromY: 6, ToX: 5, ToY: 1}},
+		{"PressKey", &Action{Kind: ActionKindPressKey, Key: "enter"}},
+		{"Wait", &Action{Kind: ActionKindWait, DurationMillis: 500}},
+	} {
+		t.Run(testCase.name, func(t *testing.T) {
+			if err := verifier.PushSnapshot(SnapshotInput{
+				Snapshots:  Snapshots{},
+				LastAction: testCase.action,
+			}); err != nil {
+				t.Fatal(err)
+			}
+			handle := verifier.runtime.GlobalObject().Get("last").ToObject(verifier.runtime)
+			goja := handle.Get("current").String()
+			web := string(EncodeLastAction(testCase.action))
+			if goja != web {
+				t.Errorf("the two hosts disagree on state.lastAction\n goja: %s\n  web: %s", goja, web)
+			}
+		})
+	}
+}