Summary
Running the app with flutter run -d web-server renders a blank page on first load. The Dart code does start (Sentry initialises, main() runs), but nothing paints until the page is manually reloaded, after which it renders and behaves normally.
The cause is an unhandled exception inside DWDS's injected debug client:
Unhandled error detected in the injected client.js script.
You can disable this script in webdev by passing --no-injected-client if it
is preventing your app from loading, but note that this will also prevent
all debugging and hot reload/restart functionality from working.
TypeError: Instance of '_JsonMap': type '_JsonMap' is not a subtype of type 'List<Object?>'
at BuiltJsonSerializers._deserialize$3 (…/dwds/src/injected/client.js:20953:36)
at Object._deserializeEvent (…/dwds/src/injected/client.js:9920:35)
This is an upstream dart-lang/webdev bug, not application code — filing here so it's documented rather than rediscovered.
Impact
Low severity, high confusion. It looks exactly like "the app is broken" or "the backend is unreachable":
- Page is blank white, no error shown
- Console shows only
DDC is about to load 1/2 scripts and appears to hang
- All assets return 200, so the network tab looks healthy
It bites hardest in automated/E2E contexts, where a blank first load can be misread as a genuine app failure. Relevant to prompts/testing/ which drives Flutter web via Chrome DevTools MCP.
Reproduction
flutter run -d web-server --web-hostname 127.0.0.1 --web-port 3000
# open http://localhost:3000 -> blank page
# reload -> app renders normally
Observed with Flutter 3.44.1 / Chrome 150.
Workarounds
- Reload the page once after first load. Simplest, keeps hot restart working.
flutter run -d chrome instead — did not exhibit the stall in testing.- For E2E runs where debugging isn't needed, serve a release build (
flutter build web --release + any static server); no DWDS, no DDC, and it loads considerably faster.
Related gotcha worth recording
When driving Flutter web through Chrome DevTools:
- The a11y snapshot is empty until semantics are enabled, because Flutter renders to canvas. Click the
<flt-semantics-placeholder> element (aria-label "Enable accessibility") first, then the tree exposes real textboxes/buttons.
- Filling a text field without clearing it first can leave a partial value — a 15-character password landed as 12 characters and produced a spurious
401 from a perfectly valid credential. Clear the field, then fill.
Both are worth adding to prompts/testing/README.md, which already notes Chrome DevTools MCP interaction as the main thing slowing prompt execution.
Summary
Running the app with
flutter run -d web-serverrenders a blank page on first load. The Dart code does start (Sentry initialises,main()runs), but nothing paints until the page is manually reloaded, after which it renders and behaves normally.The cause is an unhandled exception inside DWDS's injected debug client:
This is an upstream
dart-lang/webdevbug, not application code — filing here so it's documented rather than rediscovered.Impact
Low severity, high confusion. It looks exactly like "the app is broken" or "the backend is unreachable":
DDC is about to load 1/2 scriptsand appears to hangIt bites hardest in automated/E2E contexts, where a blank first load can be misread as a genuine app failure. Relevant to
prompts/testing/which drives Flutter web via Chrome DevTools MCP.Reproduction
Observed with Flutter 3.44.1 / Chrome 150.
Workarounds
flutter run -d chromeinstead — did not exhibit the stall in testing.flutter build web --release+ any static server); no DWDS, no DDC, and it loads considerably faster.Related gotcha worth recording
When driving Flutter web through Chrome DevTools:
<flt-semantics-placeholder>element (aria-label "Enable accessibility") first, then the tree exposes real textboxes/buttons.401from a perfectly valid credential. Clear the field, then fill.Both are worth adding to
prompts/testing/README.md, which already notes Chrome DevTools MCP interaction as the main thing slowing prompt execution.