Fix SplitPane RangeError when child count changes between rebuilds (#9822)

* Fix SplitPane RangeError when child count changes between rebuilds

Fixes #9648.

SplitPane cached its fractions list in initState only. When the parent
rebuilt the widget with a different number of children (for example,
toggling a panel via a collection-if), fractions.length stayed at the
old value while widget.minSizes and widget.children shrank, causing
minSizeForIndex to read past the end of widget.minSizes and throw
'RangeError (index): Index out of range: index should be less than 2: 2'
from the layout pass.

This adds didUpdateWidget to _SplitPaneState. When the child count
changes, fractions is reset to List.of(widget.initialFractions) so it
stays in sync with the new children and minSizes. The existing
constructor assertion already guarantees children.length matches
initialFractions.length.

Bumps devtools_app_shared to 0.5.2 with a CHANGELOG entry, and adds
regression tests that pump a 3-child SplitPane and then a 2-child
SplitPane (and vice versa) and assert no exception is thrown.

* Add release note entry for SplitPane RangeError fix (#9822)

* Address review: bump devtools_app_shared CHANGELOG to 0.5.2-wip

* Address review: bump devtools_app_shared pubspec to 0.5.2-wip

Author: ishaquehassan

packages/devtools_app/release_notes/NEXT_RELEASE_NOTES.md

@@ -15,7 +15,10 @@ To learn more about DevTools, check out the
 
 ## General updates
 
-TODO: Remove this section if there are not any updates.
+* Fixed a `RangeError` thrown by `SplitPane` when the parent rebuilt the
+  widget with a different number of children, for example when toggling a
+  panel in or out of the layout. -
+  [#9822](https://github.com/flutter/devtools/pull/9822)
 
 ## Inspector updates
 

packages/devtools_app_shared/CHANGELOG.md

@@ -3,6 +3,10 @@ Copyright 2025 The Flutter Authors
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file or at https://developers.google.com/open-source/licenses/bsd.
 -->
+## 0.5.2-wip
+* Fix a `RangeError` thrown by `SplitPane` when the number of children
+  changes between rebuilds.
+
 ## 0.5.1
 * Add DevTools-styled text field `DevToolsTextField`.
 * Updates `devtools_shared` constraint to `^13.0.0`.

packages/devtools_app_shared/lib/src/ui/split_pane.dart

@@ -88,7 +88,7 @@ final class SplitPane extends StatefulWidget {
 }
 
 final class _SplitPaneState extends State<SplitPane> {
-  late final List<double> fractions;
+  late List<double> fractions;
 
   bool get isHorizontal => widget.axis == Axis.horizontal;
 
@@ -98,6 +98,18 @@ final class _SplitPaneState extends State<SplitPane> {
     fractions = List.of(widget.initialFractions);
   }
 
+  @override
+  void didUpdateWidget(SplitPane oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    // When the number of children changes, the previously stored [fractions]
+    // list will be out of sync with [widget.minSizes] and [widget.children],
+    // which causes a RangeError during layout. Reset to the new
+    // [initialFractions] when the child count changes.
+    if (oldWidget.children.length != widget.children.length) {
+      fractions = List.of(widget.initialFractions);
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     return LayoutBuilder(builder: _buildLayout);

packages/devtools_app_shared/pubspec.yaml

@@ -3,7 +3,7 @@
 # found in the LICENSE file or at https://developers.google.com/open-source/licenses/bsd.
 name: devtools_app_shared
 description: Package of Dart & Flutter structures shared between devtools_app and devtools extensions.
-version: 0.5.1
+version: 0.5.2-wip
 repository: https://github.com/flutter/devtools/tree/master/packages/devtools_app_shared
 
 environment:

packages/devtools_app_shared/test/ui/split_pane_test.dart

@@ -1154,6 +1154,67 @@ void main() {
       );
     });
 
+    group('rebuilds with a different number of children', () {
+      testWidgets(
+        'does not throw a RangeError when child count shrinks',
+        (WidgetTester tester) async {
+          final threeChildSplit = buildSplitPane(
+            Axis.horizontal,
+            children: const [_w1, _w2, _w3],
+            initialFractions: const [0.2, 0.4, 0.4],
+            minSizes: const [50.0, 50.0, 50.0],
+          );
+          await tester.pumpWidget(wrap(threeChildSplit));
+          expect(find.byKey(_k1), findsOneWidget);
+          expect(find.byKey(_k2), findsOneWidget);
+          expect(find.byKey(_k3), findsOneWidget);
+
+          final twoChildSplit = buildSplitPane(
+            Axis.horizontal,
+            children: const [_w1, _w2],
+            initialFractions: const [0.5, 0.5],
+            minSizes: const [50.0, 50.0],
+          );
+          await tester.pumpWidget(wrap(twoChildSplit));
+          await tester.pumpAndSettle();
+
+          expect(tester.takeException(), isNull);
+          expect(find.byKey(_k1), findsOneWidget);
+          expect(find.byKey(_k2), findsOneWidget);
+          expect(find.byKey(_k3), findsNothing);
+        },
+      );
+
+      testWidgets(
+        'does not throw a RangeError when child count grows',
+        (WidgetTester tester) async {
+          final twoChildSplit = buildSplitPane(
+            Axis.horizontal,
+            children: const [_w1, _w2],
+            initialFractions: const [0.5, 0.5],
+            minSizes: const [50.0, 50.0],
+          );
+          await tester.pumpWidget(wrap(twoChildSplit));
+          expect(find.byKey(_k1), findsOneWidget);
+          expect(find.byKey(_k2), findsOneWidget);
+
+          final threeChildSplit = buildSplitPane(
+            Axis.horizontal,
+            children: const [_w1, _w2, _w3],
+            initialFractions: const [0.2, 0.4, 0.4],
+            minSizes: const [50.0, 50.0, 50.0],
+          );
+          await tester.pumpWidget(wrap(threeChildSplit));
+          await tester.pumpAndSettle();
+
+          expect(tester.takeException(), isNull);
+          expect(find.byKey(_k1), findsOneWidget);
+          expect(find.byKey(_k2), findsOneWidget);
+          expect(find.byKey(_k3), findsOneWidget);
+        },
+      );
+    });
+
     group('axisFor', () {
       testWidgetsWithWindowSize(
         'return Axis.horizontal',