Allow SliverCrossAxisGroup children to lay out with zero cross-axis extent instead of asserting by zemanux · Pull Request #191287 · flutter/flutter

zemanux · 2026-08-18T17:47:47Z

Fixes #191275

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This pull request introduces the allowHeadlessExecution property to the iOS Flutter engine and verifies its propagation from FlutterViewController via key-value coding. Additionally, it updates RenderSliverCrossAxisGroup to allow flexible children to be laid out with zero extent when constrained slivers consume the entire cross-axis extent, removing the previous assertion. Feedback suggests clamping the calculated cross-axis extent to a minimum of 0.0 to prevent negative values from triggering assertion failures in SliverConstraints when constrained children exceed the available extent, and recommends adding a test case to verify this behavior.

victorsanni

Thanks for the PR, approach looks good, left some comments.

victorsanni · 2026-10-15T00:43:50Z

      if (flex != 0) {
-        childExtent = extentPerFlexValue * flex;
-        assert(_assertOutOfExtent(childExtent));
+        // Flexible children are allowed to be laid out with zero extent without asserting.


Do the public API docs also reflect this behavior change?

Also the comment explains the diff, not the code. Better is:

// The constrained children may have consumed all of the available cross // axis extent, in which case flexible children are laid out with zero // extent, like Expanded children in a Flex. assert(extentPerFlexValue >= 0.0 && extentPerFlexValue.isFinite); // and a replacement assert to sanity check

victorsanni · 2026-10-15T00:47:13Z

After this PR, _assertOutOfExtent only fires for a constrained child when remainingExtent is exactly 0. A constrained child that is partially squished passes silently, so the hint "make sure the total … does not exceed the cross axis extent" is already inaccurate.

The condition is reachable in legitimate layouts. For example, a SliverCrossAxisGroup with a single SliverConstrainedCrossAxis throws in debug mode whenever its viewport animates/collapses to width 0. RenderFlex never aborts layout for overflow, it reports it during paint.

If we are removing the flex callsite, should we also consider removing the constrained callsite as well?

Makes sense. I removed the assertion for constrained children too, updated the docs, and added new tests in the latest commit.

victorsanni · 2026-10-15T00:49:11Z

+  testWidgets(
+    'Expanded slivers are laid out with zero extent when constrained slivers use it all up',
+    (WidgetTester tester) async {
+      final errors = <FlutterErrorDetails>[];


The FlutterError.onError override in the new test is unnecessary and hides failures.

victorsanni · 2026-10-15T00:49:55Z

-    final FlutterExceptionHandler? oldHandler = FlutterError.onError;
-    FlutterError.onError = (FlutterErrorDetails error) => errors.add(error);
+  testWidgets(
+    'Expanded slivers are laid out with zero extent when constrained slivers use it all up',


Nit: more terse title, like

Flexible slivers get zero extent when constrained slivers use all space

victorsanni

The PR title is now outdated?

zemanux · 2026-10-21T15:32:47Z

The PR title is now outdated?

I updated the title

victorsanni

LGTM

victorsanni · 2026-10-29T18:38:59Z

There is a failing customer test, rerunning to deflake.

| 00:24 +661: /b/s/w/ir/x/t/flutter_customer_testing.flame.MSJKQV/tests/packages/flame/test/game/game_widget/game_widget_lifecycle_test.dart: Game Widget - Lifecycle all events are executed in the correct order
| ══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
| The following TestFailure was thrown running a test:
| Expected: [
|             'onGameResize',
|             'onLoad',
|             'onMount',
|             'update',
|             'render',
|             'update',
|             'render',
|             'update',
|             'onRemove',
|             'onDispose'
|           ]
|   Actual: [
|             'onGameResize',
|             'onLoad',
|             'onMount',
|             'update',
|             'render',
|             'update',
|             'update',
|             'onRemove',
|             'onDispose'
|           ]
|    Which: at location [6] is 'update' instead of 'render'
|
| When the exception was thrown, this was the stack:
| #4      main.<anonymous closure>.<anonymous closure> (file:///b/s/w/ir/x/t/flutter_customer_testing.flame.MSJKQV/tests/packages/flame/test/game/game_widget/game_widget_lifecycle_test.dart:272:7)
| <asynchronous suspension>
| #5      testWidgets.<anonymous closure>.<anonymous closure> (package:flutter_test/src/widget_tester.dart:192:15)
| <asynchronous suspension>
| #6      TestWidgetsFlutterBinding._runTestBody (package:flutter_test/src/binding.dart:2062:5)
| <asynchronous suspension>
| <asynchronous suspension>
| (elided one frame from package:stack_trace)
|
| This was caught by the test expectation on the following line:
|   file:///b/s/w/ir/x/t/flutter_customer_testing.flame.MSJKQV/tests/packages/flame/test/game/game_widget/game_widget_lifecycle_test.dart line 272
| The test description was:
|   all events are executed in the correct order
| ════════════════════════════════════════════════════════════════════════════════════════════════════

…nstead of asserting

…tent

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github-actions Bot added framework flutter/packages/flutter repository. See also f: labels. f: scrolling Viewports, list views, slivers, etc. labels Aug 18, 2026

mbcorona mentioned this pull request Aug 18, 2026

SliverCrossAxisGroup throws _assertOutOfExtent when available cross-axis extent is fully consumed by inflexible children (SliverConstrainedCrossAxis) #191275

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zemanux mentioned this pull request Aug 19, 2026

Let SliverCrossAxisGroup collapse flexible children to zero extent #191373

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zemanux force-pushed the sliver_cross_axis_group branch from 075157a to 7bff7ae Compare August 25, 2026 17:44

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Comment thread packages/flutter/lib/src/rendering/sliver_group.dart Resolved

Comment thread packages/flutter/test/widgets/sliver_cross_axis_group_test.dart Resolved

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Comment thread packages/flutter/test/widgets/sliver_cross_axis_group_test.dart Resolved

Comment thread packages/flutter/lib/src/rendering/sliver_group.dart Resolved

zemanux changed the title ~~Fix lay out SliverCrossAxisGroup flexible children with zero extent i…~~ Allow SliverCrossAxisGroup children to lay out with zero cross-axis extent instead of asserting Oct 21, 2026

zemanux requested a review from victorsanni October 21, 2026 15:33

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zemanux and others added 4 commits October 31, 2026 10:34

Fix lay out SliverCrossAxisGroup flexible children with zero extent i…

8ae9f87

…nstead of asserting

Review - added new tests and docs

4ef9d95

Review - allow constrained slivers to lay out with zero cross-axis ex…

de8aef1

…tent

Review - fix comments and test

a916934

zemanux force-pushed the sliver_cross_axis_group branch from bbc7b2d to a916934 Compare October 31, 2026 09:34

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zemanux self-assigned this Oct 31, 2026

Merge branch 'master' into sliver_cross_axis_group

c4d9b77

victorsanni added CICD Run CI/CD autosubmit Merge PR when tree becomes green via auto submit App labels Nov 11, 2026

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Merged via the queue into flutter:master with commit 3b3db93 Nov 11, 2026
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Roll Flutter from b98afb524077 to 9c427b5d7b1c (29 revisions) flutter/packages#13636

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justinmc mentioned this pull request Nov 13, 2026

Flutter Notable Commits - 2026 #181433

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zemanux commented Aug 18, 2026

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