[ndonkoHenri]

Fixes #6993.

On desktop (and mobile), flet test runs the app inside the Flutter test host on its embedded Python. runFletDeviceTest started the app, pumped a fixed 20 frames (~2 s), then connected the remote tester. When the app took longer than that to start, the test ran against a blank window. The first run after a build is that slow case, especially on Windows where antivirus scans the new bundle, so a freshly created project failed its own sample test.

The driver now pumps until Python sends a test-only test_ready page flag after main() finishes its initial updates (or after the first yield of a generator handler). Registration clears isLoading before main() runs, so registration alone does not unblock the tester. Empty apps also signal readiness. The driver times out after 2 minutes with a clear message and fails with the app's startup error if the backend reports one.

Testing

Automated follow-up validation after merging the current flet-1.1.0 target:

• Python unit suite: 443 passed, 9 skipped, including delayed initialization, empty apps, startup failures, and sync/async generator handlers.
• Flutter startup regression tests: 5 passed, covering rendered readiness, empty apps, startup errors, and timeouts.
• Confirmed the delayed-start Flutter regression fails with the original registration-only readiness condition.
• Flutter analysis, Python lint/format checks, and commit hooks passed.

Full device builds were not rerun for the readiness follow-up. Original manual validation of the initial driver change:

Reproduced on Flet 1.0.5 with the flet create counter project, then ran the same project against the patched driver (via [tool.flet.flutter.pubspec.dev_dependencies]):

Where	Scenario	Stock driver	Patched driver
macOS	normal project	passes	passes
macOS	time.sleep(5) at the top of main.py	fails as reported (assert 0 == 1)	passes
Windows 11 VM	first cold run of an untouched project	fails as reported	passes (build/ deleted first)
Windows 11 VM	warm rerun	passes	passes

The time.sleep(5) variant makes the race reproducible on any OS. flutter analyze on flet_integration_test is clean.

Summary by Sourcery

Ensure device-mode Flet tests synchronize with application startup before executing remote test commands.

Bug Fixes:

• Make flet test wait for the app’s first page to finish initializing before connecting the remote tester, preventing failures for slow-starting applications.
• Report application startup errors and timeout failures clearly instead of running tests against an uninitialized app.

Enhancements:

• Add readiness signaling for test-mode sessions, including empty and generator-based applications.

Tests:

• Add Flutter and Python coverage for delayed startup, readiness signaling, startup errors, timeouts, empty apps, and generator-based apps.

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