Fix scrollUntilVisible in WidgetTester (#159582)

Fixes #143921.

The single `moveBy` event may be consumed when `onTap` is present.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

packages/flutter_test/lib/src/controller.dart

@@ -2312,6 +2312,10 @@ abstract class WidgetController {
   /// If `scrollable` is `null`, a [Finder] that looks for a [Scrollable] is
   /// used instead.
   ///
+  /// If `continuous` is `true`, the gesture will be reused to simulate the effect
+  /// of actual finger scrolling, which is useful when used alongside listeners
+  /// like [GestureDetector.onTap]. The default is `false`.
+  ///
   /// Throws a [StateError] if `finder` is not found after `maxScrolls` scrolls.
   ///
   /// This is different from [ensureVisible] in that this allows looking for
@@ -2328,6 +2332,7 @@ abstract class WidgetController {
     finders.FinderBase<Element>? scrollable,
     int maxScrolls = 50,
     Duration duration = const Duration(milliseconds: 50),
+    bool continuous = false,
   }) {
     assert(maxScrolls > 0);
     scrollable ??= finders.find.byType(Scrollable);
@@ -2349,6 +2354,7 @@ abstract class WidgetController {
         moveStep,
         maxIteration: maxScrolls,
         duration: duration,
+        continuous: continuous,
       );
     });
   }
@@ -2370,13 +2376,21 @@ abstract class WidgetController {
     Offset moveStep, {
     int maxIteration = 50,
     Duration duration = const Duration(milliseconds: 50),
+    bool continuous = false,
   }) {
     return TestAsyncUtils.guard<void>(() async {
+      TestGesture? gesture;
       while (maxIteration > 0 && finder.evaluate().isEmpty) {
-        await drag(view, moveStep);
+        if (continuous) {
+          gesture ??= await startGesture(getCenter(view, warnIfMissed: true));
+          await gesture.moveBy(moveStep);
+        } else {
+          await drag(view, moveStep);
+        }
         await pump(duration);
         maxIteration -= 1;
       }
+      await gesture?.up();
       await Scrollable.ensureVisible(element(finder));
     });
   }

packages/flutter_test/test/controller_test.dart

@@ -612,6 +612,44 @@ void main() {
     });
   });
 
+  // Regression test for https://github.com/flutter/flutter/issues/143921.
+  testWidgets('WidgetTester.scrollUntilVisible should work together with onTap', (
+    WidgetTester tester,
+  ) async {
+    const int itemCount = 20;
+
+    Widget buildFrame(bool hasOnTap) {
+      return MaterialApp(
+        home: Scaffold(
+          body: ListView.builder(
+            key: ValueKey<bool>(hasOnTap), // Trigger a rebuild.
+            itemCount: itemCount,
+            itemBuilder: (BuildContext context, int index) {
+              return ListTile(onTap: hasOnTap ? () {} : null, title: Text('$index'));
+            },
+          ),
+        ),
+      );
+    }
+
+    final Finder target = find.text('${itemCount - 1}');
+    final Finder scrollable = find.byType(Scrollable);
+
+    // Scroll without onTap.
+    await tester.pumpWidget(buildFrame(false));
+    await tester.pumpAndSettle();
+    expect(target, findsNothing);
+    await tester.scrollUntilVisible(target, 20, scrollable: scrollable, continuous: true);
+    expect(target, findsOneWidget);
+
+    // Scroll with onTap.
+    await tester.pumpWidget(buildFrame(true));
+    await tester.pumpAndSettle();
+    expect(target, findsNothing);
+    await tester.scrollUntilVisible(target, 20, scrollable: scrollable, continuous: true);
+    expect(target, findsOneWidget);
+  });
+
   testWidgets('platformDispatcher exposes the platformDispatcher from binding', (
     WidgetTester tester,
   ) async {