Improve some scrollbar error messages (#143279)

Adds some missing spaces, rewords some errors, and splits some errors into more lines.

packages/flutter/lib/src/widgets/scrollbar.dart

@@ -1466,18 +1466,24 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
           ErrorHint(
             'The Scrollbar attempted to use the $controllerForError. This '
             'ScrollController should be associated with the ScrollView that '
-            'the Scrollbar is being applied to.'
-            '${tryPrimary
-              ? 'When ScrollView.scrollDirection is Axis.vertical on mobile '
-                'platforms will automatically use the '
-                'PrimaryScrollController if the user has not provided a '
-                'ScrollController. To use the PrimaryScrollController '
-                'explicitly, set ScrollView.primary to true for the Scrollable '
-                'widget.'
-              : 'When providing your own ScrollController, ensure both the '
-                'Scrollbar and the Scrollable widget use the same one.'
-            }',
+            'the Scrollbar is being applied to.',
           ),
+          if (tryPrimary) ...<ErrorHint>[
+            ErrorHint(
+              'If a ScrollController has not been provided, the '
+              'PrimaryScrollController is used by default on mobile platforms '
+              'for ScrollViews with an Axis.vertical scroll direction.',
+            ),
+            ErrorHint(
+              'To use the PrimaryScrollController explicitly, '
+              'set ScrollView.primary to true on the Scrollable widget.',
+            ),
+          ]
+          else
+            ErrorHint(
+              'When providing your own ScrollController, ensure both the '
+              'Scrollbar and the Scrollable widget use the same one.',
+            ),
         ]);
       }
       return true;
@@ -1491,26 +1497,32 @@ class RawScrollbarState<T extends RawScrollbar> extends State<T> with TickerProv
         }
         throw FlutterError.fromParts(<DiagnosticsNode>[
           ErrorSummary(
-            'The $controllerForError is currently attached to more than one '
-            'ScrollPosition.',
+            'The $controllerForError is attached to more than one ScrollPosition.',
           ),
           ErrorDescription(
             'The Scrollbar requires a single ScrollPosition in order to be painted.',
           ),
           ErrorHint(
             'When $when, the associated ScrollController must only have one '
-            'ScrollPosition attached.'
-            '${tryPrimary
-              ? 'If a ScrollController has not been provided, the '
-                'PrimaryScrollController is used by default on mobile platforms '
-                'for ScrollViews with an Axis.vertical scroll direction. More '
-                'than one ScrollView may have tried to use the '
-                'PrimaryScrollController of the current context. '
-                'ScrollView.primary can override this behavior.'
-              : 'The provided ScrollController must be unique to one '
-                'ScrollView widget.'
-            }',
+            'ScrollPosition attached.',
           ),
+          if (tryPrimary) ...<ErrorHint>[
+            ErrorHint(
+              'If a ScrollController has not been provided, the '
+              'PrimaryScrollController is used by default on mobile platforms '
+              'for ScrollViews with an Axis.vertical scroll direction.'
+            ),
+            ErrorHint(
+              'More than one ScrollView may have tried to use the '
+              'PrimaryScrollController of the current context. '
+              'ScrollView.primary can override this behavior.'
+            ),
+          ]
+          else
+            ErrorHint(
+              'The provided ScrollController cannot be shared by multiple '
+              'ScrollView widgets.'
+            ),
         ]);
       }
       return true;

packages/flutter/test/material/scrollbar_test.dart

@@ -1810,7 +1810,12 @@ void main() {
     FlutterError error = tester.takeException() as FlutterError;
     expect(
       error.message,
-      contains('The PrimaryScrollController is currently attached to more than one ScrollPosition.'),
+      '''
+The PrimaryScrollController is attached to more than one ScrollPosition.
+The Scrollbar requires a single ScrollPosition in order to be painted.
+When Scrollbar.thumbVisibility is true, the associated ScrollController must only have one ScrollPosition attached.
+If a ScrollController has not been provided, the PrimaryScrollController is used by default on mobile platforms for ScrollViews with an Axis.vertical scroll direction.
+More than one ScrollView may have tried to use the PrimaryScrollController of the current context. ScrollView.primary can override this behavior.''',
     );
 
     // Asserts when using the ScrollController provided by the user.
@@ -1829,7 +1834,11 @@ void main() {
     error = tester.takeException() as FlutterError;
     expect(
       error.message,
-      contains('The provided ScrollController is currently attached to more than one ScrollPosition.'),
+      '''
+The provided ScrollController is attached to more than one ScrollPosition.
+The Scrollbar requires a single ScrollPosition in order to be painted.
+When Scrollbar.thumbVisibility is true, the associated ScrollController must only have one ScrollPosition attached.
+The provided ScrollController cannot be shared by multiple ScrollView widgets.''',
     );
 
     scrollController.dispose();

packages/flutter/test/widgets/scrollbar_test.dart

@@ -1514,7 +1514,63 @@ void main() {
     expect(exception, isAssertionError);
     expect(
       exception.message,
-      contains("The Scrollbar's ScrollController has no ScrollPosition attached."),
+      '''
+The Scrollbar's ScrollController has no ScrollPosition attached.
+A Scrollbar cannot be painted without a ScrollPosition.
+The Scrollbar attempted to use the PrimaryScrollController. This ScrollController should be associated with the ScrollView that the Scrollbar is being applied to.
+If a ScrollController has not been provided, the PrimaryScrollController is used by default on mobile platforms for ScrollViews with an Axis.vertical scroll direction.
+To use the PrimaryScrollController explicitly, set ScrollView.primary to true on the Scrollable widget.''',
+    );
+  });
+
+  testWidgets('Scrollbars assert on multiple scroll positions', (WidgetTester tester) async {
+    final ScrollController scrollController = ScrollController();
+    addTearDown(scrollController.dispose);
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(),
+          child: PrimaryScrollController(
+            controller: scrollController,
+            child: Row(
+              children: <Widget>[
+                RawScrollbar(
+                  controller: scrollController,
+                  child: const SingleChildScrollView(
+                    child: SizedBox(width: 10.0, height: 4000.0),
+                  ),
+                ),
+                RawScrollbar(
+                  controller: scrollController,
+                  child: const SingleChildScrollView(
+                    child: SizedBox(width: 10.0, height: 4000.0),
+                  ),
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    AssertionError? exception = tester.takeException() as AssertionError?;
+    // The scrollbar is not visible and cannot be interacted with, so no assertion.
+    expect(exception, isNull);
+    // Scroll to trigger the scrollbar to come into view.
+    final Finder scrollViews = find.byType(SingleChildScrollView);
+    final TestGesture gesture = await tester.startGesture(tester.getCenter(scrollViews.first));
+    await gesture.moveBy(const Offset(0.0, -20.0));
+    exception = tester.takeException() as AssertionError;
+    expect(exception, isAssertionError);
+    expect(
+      exception.message,
+      '''
+The provided ScrollController is attached to more than one ScrollPosition.
+The Scrollbar requires a single ScrollPosition in order to be painted.
+When the scrollbar is interactive, the associated ScrollController must only have one ScrollPosition attached.
+The provided ScrollController cannot be shared by multiple ScrollView widgets.''',
     );
   });
 
@@ -2892,7 +2948,7 @@ void main() {
     expect(scrollController.offset, greaterThan(lastPosition));
   });
 
-  testWidgets('The bar support mouse wheel event', (WidgetTester tester) async {
+  testWidgets('The bar supports mouse wheel event', (WidgetTester tester) async {
     // Regression test for https://github.com/flutter/flutter/pull/109659
     final ScrollController scrollController = ScrollController();
     addTearDown(scrollController.dispose);