test(browser): assert an uncaught page exception reaches the trace

the page buffered its uncaught errors in v8 and nothing carried them out, so state.exceptions was empty on the host and no trace held one, leaving an offline crash oracle nothing to read. asserts the recorded trace steps rather than the summary.

test/browser/browser_test.go

@@ -10,9 +10,12 @@
 package browser_test
 
 import (
+	"bufio"
 	"context"
+	"encoding/json"
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"path/filepath"
 	"runtime"
 	"slices"
@@ -82,6 +85,14 @@ func TestBrowserAriaRoleIsTappable(t *testing.T) {
 // and returns every property name that was ever reported violated.
 func runFixture(t *testing.T, name string) []string {
 	t.Helper()
+	violations, _ := runFixtureRecording(t, name)
+	return violations
+}
+
+// runFixtureRecording is runFixture plus the run directory, for a test that
+// asserts what the run left on disk rather than what it reported.
+func runFixtureRecording(t *testing.T, name string) ([]string, string) {
+	t.Helper()
 
 	server := httptest.NewServer(http.FileServer(http.Dir(testdataDir(t))))
 	t.Cleanup(server.Close)
@@ -139,12 +150,15 @@ func runFixture(t *testing.T, name string) []string {
 	if err != nil {
 		t.Fatalf("run: %v", err)
 	}
+	if err := traceWriter.Close(); err != nil {
+		t.Fatalf("close trace: %v", err)
+	}
 
 	var violated []string
 	for _, record := range summary.Violations {
 		violated = append(violated, record.Properties...)
 	}
-	return violated
+	return violated, traceWriter.Directory()
 }
 
 // bundleSpec compiles the fixture spec for both runtimes: the goja bundle the
@@ -222,3 +236,42 @@ func TestBrowserUndefinedExtractorStaysUndefined(t *testing.T) {
 		t.Fatalf("nothing was ever tapped, so the property above held vacuously; violations=%v", violations)
 	}
 }
+
+// TestBrowserUncaughtExceptionReachesTheTrace covers the error surface a web
+// run has no other source for. The page buffers its uncaught errors in V8;
+// nothing used to carry them out, so state.exceptions was empty on the host
+// and no trace ever held one, leaving an offline crash oracle nothing to read.
+func TestBrowserUncaughtExceptionReachesTheTrace(t *testing.T) {
+	violations, directory := runFixtureRecording(t, "throwing")
+	if !slices.Contains(violations, "noUncaughtExceptions") {
+		t.Fatalf("the page never threw; violations=%v", violations)
+	}
+
+	file, err := os.Open(filepath.Join(directory, "trace.jsonl"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer file.Close()
+	scanner := bufio.NewScanner(file)
+	scanner.Buffer(make([]byte, 0, 64*1024), 8*1024*1024)
+	var recorded []trace.Exception
+	for scanner.Scan() {
+		var step trace.Step
+		if err := json.Unmarshal(scanner.Bytes(), &step); err != nil {
+			t.Fatalf("decode step: %v", err)
+		}
+		if step.TraceVersion != trace.TraceVersion {
+			t.Errorf("step %d: trace_version = %d, want %d", step.Index, step.TraceVersion, trace.TraceVersion)
+		}
+		recorded = append(recorded, step.Exceptions...)
+	}
+	if err := scanner.Err(); err != nil {
+		t.Fatal(err)
+	}
+	if len(recorded) == 0 {
+		t.Fatal("the page threw and the verdict saw it, but no trace step carries an exception")
+	}
+	if recorded[0].Class == "" || recorded[0].Message == "" {
+		t.Errorf("exception recorded without class or message: %+v", recorded[0])
+	}
+}