Fix integration test failures caused by render overflow errors

[FeodorFitsner]

Problem

Several macOS integration test suites were failing deterministically (extensions, controls/material, examples/controls/material, examples/controls/core) with:

RuntimeError: Flutter integration test process failed with exit code 1. See the Flutter test output above.

...while all host-side assertions and golden screenshots passed, and "the Flutter test output above" was empty.

Root cause: any FlutterError reported during a frame — typically a benign RenderFlex overflowed — fails the on-device testWidgets body, which the exit-code check from #6623 surfaces as a teardown error. Diagnosis was doubly hard because:

• CI discarded Flutter process output entirely (DEVNULL unless FLET_TEST_VERBOSE=1), and even locally flutter test never prints the failing exception — only "Test failed. See exception logs above."
• The module-scoped flet_app fixture blames the last test in the module; the real culprit can be any test in it (grid_view's culprit was test_basic, not test_horizontal; markdown's was test_md_1, not test_md_3).

The five deterministic failures were all layout overflows: grid_view test_basic (200px, page-wide 780×780 grid item), markdown test_md_1 (48px), spinkit test_all_controls_render (867px, 30 spinners), row test_vertical_alignment (9px — caused by the screenshot wrapper's own margins), datatable test_sortable_and_selectable (2px — the example's table is 582px tall vs 580 available, overflowing on the very first frame at the default 800×600 window).

Changes

Test harness — diagnosability:

• FletTestApp captures Flutter test process output (line-truncated, tail-bounded) and dumps it when the process exits non-zero, so CI failures show the actual reason.
• host_test.dart hooks reportTestException to print the exception and stack, and delays tearDownAll by 2s so end-of-run reports aren't lost in the process-exit race.

Test harness — overflow prevention:

• assert_control_screenshot / wrap_page_controls_in_screenshot host the Screenshot control in a hidden-scrollbar scrollable column, so content taller than the test window scrolls instead of overflowing. The child sees the same unbounded-height constraints as before, so captures — and all existing goldens — are unaffected. Skipped when the page is already scrollable (an expanded host inside a scrollable page would be flex-in-unbounded) and for expand_screenshot=True.

Tests and examples:

• test_spinkit: render the wrapped row of all spinners inside a scrollable column.
• data_table/sortable_and_selectable example: page.scroll = AUTO — the example genuinely overflowed by 2px on startup at the default window size (visible as debug stripes with flet run).
• Regenerated stale client/pubspec.lock (was missing flet_integration_test, wrong screen_brightness).

No golden images changed.

Verification

All five previously failing modules pass locally on macOS against unchanged goldens, plus sanity checks for test_slider::test_handling_events (wrap + absolute-coordinate taps) and test_tabs (expand_screenshot=True path). Full CI matrix was green on this branch with the fixes; the temporarily trimmed matrix used during iteration has been restored.

Notes

The intermittent "Flutter process exited early with code 1/79" startup flakes seen in some runs (range_slider, textfield) are a separate issue — the new output capture will reveal their cause the next time they occur.

Summary by Sourcery

Improve integration test reliability by capturing Flutter test output and preventing layout overflows in screenshot hosts and examples.

New Features:

• Add buffered capture and tail-dump of Flutter integration test process output when the process exits with a non-zero code.

Bug Fixes:

• Prevent layout overflow errors from causing deterministic failures in integration tests by hosting screenshot content and spinner rows in scrollable containers.
• Fix the sortable-and-selectable data table example overflowing the default window size by enabling automatic page scrolling.

Enhancements:

• Install a custom exception reporter in the Flutter host test to log failing exceptions and stacks, and delay teardown to ensure logs are flushed.
• Make screenshot helpers wrap content in a hidden-scrollbar scrollable column when the page itself is not scrollable, keeping captures stable while avoiding overflows.

Build:

• Regenerate the client pubspec.lock to reflect current dependencies including flet_integration_test and correct screen_brightness versions.

Tests:

• Adjust the spinkit integration test to render all spinners inside a scrollable column instead of a tall wrapped row to avoid page overflow.

[sourcery-ai]

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