diff --git a/internal/driver/mock/mock.go b/internal/driver/mock/mock.go
index 07adadc..5751c89 100644
--- a/internal/driver/mock/mock.go
+++ b/internal/driver/mock/mock.go
@@ -55,6 +55,12 @@ type Driver struct {
 	LogEntries    []driver.LogEntry
 	MetricsData   driver.Metrics
 	Failures      map[ActionKind]error
+
+	// ForegroundResults is consumed one entry per ForegroundApp call (the
+	// last entry repeats). Empty yields "", which disables the runner's
+	// app-scope guard so tests that don't care are unaffected.
+	ForegroundResults []string
+	foregroundIndex   int
 }
 
 func New() *Driver {
@@ -96,6 +102,20 @@ func (d *Driver) Launch(_ context.Context, bundleID string, clearState bool, _ m
 	return nil
 }
 
+func (d *Driver) ForegroundApp(_ context.Context) (string, error) {
+	d.mutex.Lock()
+	defer d.mutex.Unlock()
+	if len(d.ForegroundResults) == 0 {
+		return "", nil
+	}
+	index := d.foregroundIndex
+	if index >= len(d.ForegroundResults) {
+		index = len(d.ForegroundResults) - 1
+	}
+	d.foregroundIndex++
+	return d.ForegroundResults[index], nil
+}
+
 func (d *Driver) Terminate(ctx context.Context) error {
 	if err := d.failure(ActionTerminate); err != nil {
 		return err
diff --git a/internal/runner/runner.go b/internal/runner/runner.go
index bb0cb6a..1b49cf3 100644
--- a/internal/runner/runner.go
+++ b/internal/runner/runner.go
@@ -67,6 +67,14 @@ func Run(ctx context.Context, options Options) (Summary, error) {
 		stepIndex++
 		stepStart := time.Now()
 
+		// Keep exploration scoped to the app under test. If a prior action
+		// backed out of (or otherwise left) the app, relaunch it before we
+		// observe or act, so properties never evaluate against a foreign app
+		// and actions never land outside the app.
+		if ensureForeground(ctx, options, logger, stepIndex) {
+			lastAction = nil
+		}
+
 		// Hierarchy, metrics, and logs are independent device reads — run
 		// them concurrently so metrics+logs hide behind the hierarchy fetch.
 		var tree *hierarchy.Tree
@@ -263,6 +271,36 @@ func violationNames(verdicts map[string]ltl.Verdict) []string {
 	return names
 }
 
+// ensureForeground keeps the app under test in the foreground. When the driver
+// can report the foreground app and it no longer matches the bundle under test,
+// the app is relaunched. Returns true when a relaunch happened so the caller
+// can drop the now-stale lastAction. Drivers without ForegroundChecker (web,
+// iOS) are a no-op.
+func ensureForeground(ctx context.Context, options Options, logger *slog.Logger, stepIndex int) bool {
+	checker, ok := options.Driver.(driver.ForegroundChecker)
+	if !ok || options.BundleID == "" {
+		return false
+	}
+	foreground, err := checker.ForegroundApp(ctx)
+	if err != nil {
+		logger.Warn("foreground check failed", "step", stepIndex, "err", err)
+		return false
+	}
+	if foreground == "" || foreground == options.BundleID {
+		return false
+	}
+	logger.Warn("app left foreground; relaunching",
+		"step", stepIndex, "foreground", foreground, "want", options.BundleID)
+	if err := options.Driver.Launch(ctx, options.BundleID, false, nil); err != nil {
+		logger.Warn("relaunch failed", "step", stepIndex, "err", err)
+		return false
+	}
+	idleCtx, cancel := context.WithTimeout(ctx, options.IdleTimeout)
+	_ = options.Driver.WaitForIdle(idleCtx, options.IdleTimeout)
+	cancel()
+	return true
+}
+
 func applyAction(ctx context.Context, drv driver.DeviceDriver, action verifier.Action, tree *hierarchy.Tree) error {
 	switch action.Kind {
 	case verifier.ActionKindTap:
diff --git a/internal/runner/runner_test.go b/internal/runner/runner_test.go
index 2f7c54c..e06a2c9 100644
--- a/internal/runner/runner_test.go
+++ b/internal/runner/runner_test.go
@@ -426,3 +426,56 @@ func containsProperty(records []ViolationRecord, property string) bool {
 	}
 	return false
 }
+
+func TestRunner_RelaunchesWhenAppLeavesForeground(t *testing.T) {
+	state := newHarness(t)
+	// Always report a foreign app, so every step's guard must relaunch.
+	state.mock.ForegroundResults = []string{"com.android.chrome"}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	_, err := Run(ctx, Options{
+		Duration:    100 * time.Millisecond,
+		IdleTimeout: 20 * time.Millisecond,
+		BundleID:    "app.folio",
+		Driver:      state.mock,
+		Verifier:    state.verifier,
+		TraceWriter: state.writer,
+	})
+	if err != nil {
+		t.Fatalf("Run: %v", err)
+	}
+	relaunches := 0
+	for _, a := range state.mock.Actions() {
+		if a.Kind == mockdriver.ActionLaunch && a.BundleID == "app.folio" && !a.ClearState {
+			relaunches++
+		}
+	}
+	if relaunches == 0 {
+		t.Fatal("expected runner to relaunch app.folio when foreground escaped, got none")
+	}
+}
+
+func TestRunner_NoRelaunchWhenAppInForeground(t *testing.T) {
+	state := newHarness(t)
+	state.mock.ForegroundResults = []string{"app.folio"}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	_, err := Run(ctx, Options{
+		Duration:    100 * time.Millisecond,
+		IdleTimeout: 20 * time.Millisecond,
+		BundleID:    "app.folio",
+		Driver:      state.mock,
+		Verifier:    state.verifier,
+		TraceWriter: state.writer,
+	})
+	if err != nil {
+		t.Fatalf("Run: %v", err)
+	}
+	for _, a := range state.mock.Actions() {
+		if a.Kind == mockdriver.ActionLaunch {
+			t.Fatalf("expected no relaunch while app in foreground, got %v", a)
+		}
+	}
+}