parse only large transcript chunks off-isolate; deflake stream tests

The hang fix offloaded every transcript parse to Isolate.run, including
the small per-poll appends. Spawning a one-shot isolate each tick is pure
overhead and, under concurrent test load, the spawn+round-trip latency
blew the streaming tests' fixed-delay window — transcript_reader_test
flaked intermittently in the full suite.

Only chunks >= 64KB now go off-isolate (the initial-tail case that
actually janks a frame); small appends parse inline. The streaming tests
poll until the expected items arrive instead of waiting a fixed delay, so
they're robust regardless of parse latency or scheduler load.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/builtin/claude/src/transcript_reader.dart

@@ -152,6 +152,11 @@ String _shortId(String uuid) => uuid.length >= 8 ? uuid.substring(0, 8) : uuid;
 /// stream incrementally.
 const _defaultInitialTailBytes = 256 * 1024;
 
+/// Chunks at least this large are parsed in a background isolate; smaller
+/// ones parse inline. Streaming appends are small, so this keeps the
+/// off-thread parse to the initial-tail case that actually janks a frame.
+const _isolateParseThreshold = 64 * 1024;
+
 /// Known major transcript versions.
 const _knownMajorVersions = {1, 2};
 
@@ -321,9 +326,12 @@ class TranscriptReader {
     }
     if (controller.isClosed) return;
 
-    // Parse off the UI isolate — the initial chunk can be sizeable and
-    // JSON-decoding it on the main thread would jank the frame.
-    final parsed = await Isolate.run(() => parseTranscriptChunk(chunk));
+    // Parse off the UI isolate only when the chunk is big enough to jank a
+    // frame — the initial tail read (up to [_initialTailBytes]) is the case
+    // that froze the app. Streaming appends are small (a message at a time);
+    // parsing those inline avoids spawning a one-shot isolate every poll
+    // tick, which is pure overhead and adds latency under load.
+    final parsed = chunk.length >= _isolateParseThreshold ? await Isolate.run(() => parseTranscriptChunk(chunk)) : parseTranscriptChunk(chunk);
     if (controller.isClosed) return;
     for (final w in parsed.warnings) {
       _onWarn(w);

test/builtin/claude/transcript_reader_test.dart

@@ -30,6 +30,19 @@ void appendLines(File file, List<Map<String, dynamic>> lines) {
   );
 }
 
+/// Poll [ready] until it returns true or [timeout] elapses. Streaming
+/// assertions use this instead of a fixed delay so they don't flake under
+/// load (the reader polls on a timer and may parse off-isolate).
+Future<void> pumpUntil(
+  bool Function() ready, {
+  Duration timeout = const Duration(seconds: 5),
+}) async {
+  final deadline = DateTime.now().add(timeout);
+  while (!ready() && DateTime.now().isBefore(deadline)) {
+    await Future<void>.delayed(const Duration(milliseconds: 10));
+  }
+}
+
 /// JSONL envelope skeleton with default sentinel values.
 Map<String, dynamic> envelope({
   required String type,
@@ -559,8 +572,7 @@ void main() {
       final collected = <ConversationItem>[];
       final sub = reader.stream.listen(collected.add);
 
-      // Allow a few poll cycles.
-      await Future<void>.delayed(const Duration(milliseconds: 150));
+      await pumpUntil(() => collected.whereType<UserMessage>().isNotEmpty && collected.whereType<AssistantTextMessage>().isNotEmpty);
 
       await sub.cancel();
       await reader.dispose();
@@ -585,14 +597,14 @@ void main() {
       final sub = reader.stream.listen(collected.add);
 
       // Let the reader consume the initial lines.
-      await Future<void>.delayed(const Duration(milliseconds: 100));
+      await pumpUntil(() => collected.whereType<UserMessage>().isNotEmpty);
       final countAfterInit = collected.length;
 
       // Append new lines.
       appendLines(sessionFile, [assistantText('a1', 'appended reply')]);
 
       // Let the reader pick up the append.
-      await Future<void>.delayed(const Duration(milliseconds: 100));
+      await pumpUntil(() => collected.whereType<AssistantTextMessage>().isNotEmpty);
 
       await sub.cancel();
       await reader.dispose();
@@ -619,7 +631,7 @@ void main() {
       final collected = <ConversationItem>[];
       final sub = reader.stream.listen(collected.add);
 
-      await Future<void>.delayed(const Duration(milliseconds: 100));
+      await pumpUntil(() => collected.whereType<UserMessage>().any((m) => m.text == 'old session'));
 
       // Create a newer session file (ensure mtime difference with touch-like approach).
       final newerFile = File('${projectDir.path}/session-xyz.jsonl');
@@ -631,7 +643,7 @@ void main() {
       final now = DateTime.now();
       await newerFile.setLastModified(now);
 
-      await Future<void>.delayed(const Duration(milliseconds: 200));
+      await pumpUntil(() => collected.whereType<UserMessage>().any((m) => m.text == 'new session'));
 
       await sub.cancel();
       await reader.dispose();
@@ -661,7 +673,7 @@ void main() {
       final collected = <ConversationItem>[];
       final sub = reader.stream.listen(collected.add);
 
-      await Future<void>.delayed(const Duration(milliseconds: 150));
+      await pumpUntil(() => collected.isNotEmpty);
 
       await sub.cancel();
       await reader.dispose();
@@ -692,7 +704,7 @@ void main() {
       final collected = <ConversationItem>[];
       final sub = reader.stream.listen(collected.add);
 
-      await Future<void>.delayed(const Duration(milliseconds: 200));
+      await pumpUntil(() => collected.isNotEmpty);
 
       await sub.cancel();
       await reader.dispose();