Adjust selection drag start position for viewport offset changes (#80047)

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

@@ -952,7 +952,7 @@ class TextSelectionGestureDetectorBuilder {
   @protected
   final TextSelectionGestureDetectorBuilderDelegate delegate;
 
-  /// Returns true iff lastSecondaryTapDownPosition was on selection.
+  /// Returns true if lastSecondaryTapDownPosition was on selection.
   bool get _lastSecondaryTapWasOnSelection {
     assert(renderEditable.lastSecondaryTapDownPosition != null);
     if (renderEditable.selection == null) {
@@ -985,6 +985,9 @@ class TextSelectionGestureDetectorBuilder {
   @protected
   RenderEditable get renderEditable => editableText.renderEditable;
 
+  /// The viewport offset pixels of the [RenderEditable] at the last drag start.
+  double _dragStartViewportOffset = 0.0;
+
   /// Handler for [TextSelectionGestureDetector.onTapDown].
   ///
   /// By default, it forwards the tap to [RenderEditable.handleTapDown] and sets
@@ -1197,6 +1200,8 @@ class TextSelectionGestureDetectorBuilder {
       from: details.globalPosition,
       cause: SelectionChangedCause.drag,
     );
+
+    _dragStartViewportOffset = renderEditable.offset.pixels;
   }
 
   /// Handler for [TextSelectionGestureDetector.onDragSelectionUpdate].
@@ -1212,8 +1217,14 @@ class TextSelectionGestureDetectorBuilder {
   void onDragSelectionUpdate(DragStartDetails startDetails, DragUpdateDetails updateDetails) {
     if (!delegate.selectionEnabled)
       return;
+
+    // Adjust the drag start offset for possible viewport offset changes.
+    final Offset startOffset = renderEditable.maxLines == 1
+        ? Offset(renderEditable.offset.pixels - _dragStartViewportOffset, 0.0)
+        : Offset(0.0, renderEditable.offset.pixels - _dragStartViewportOffset);
+
     renderEditable.selectPositionAt(
-      from: startDetails.globalPosition,
+      from: startDetails.globalPosition - startOffset,
       to: updateDetails.globalPosition,
       cause: SelectionChangedCause.drag,
     );
@@ -1618,7 +1629,7 @@ class ClipboardStatusNotifier extends ValueNotifier<ClipboardStatus> with Widget
   }) : super(value);
 
   bool _disposed = false;
-  /// True iff this instance has been disposed.
+  /// True if this instance has been disposed.
   bool get disposed => _disposed;
 
   /// Check the [Clipboard] and update [value] if needed.

packages/flutter/test/widgets/text_selection_test.dart

@@ -624,6 +624,44 @@ void main() {
     expect(editableText.selectionOverlay!.toolbarIsVisible, isFalse);
   });
 
+  testWidgets('test TextSelectionGestureDetectorBuilder drag with RenderEditable viewport offset change', (WidgetTester tester) async {
+    await pumpTextSelectionGestureDetectorBuilder(tester);
+    final FakeRenderEditable renderEditable = tester.renderObject(find.byType(FakeEditable));
+
+    // Reconfigure the RenderEditable for multi-line.
+    renderEditable.maxLines = null;
+    renderEditable.offset = ViewportOffset.fixed(20.0);
+    renderEditable.layout(const BoxConstraints.tightFor(width: 400, height: 300.0));
+    await tester.pumpAndSettle();
+
+    final TestGesture gesture = await tester.startGesture(
+      const Offset(200.0, 200.0),
+      kind: PointerDeviceKind.mouse,
+    );
+    addTearDown(gesture.removePointer);
+    await tester.pumpAndSettle();
+    expect(renderEditable.selectPositionAtCalled, isFalse);
+
+    await gesture.moveTo(const Offset(300.0, 200.0));
+    await tester.pumpAndSettle();
+    expect(renderEditable.selectPositionAtCalled, isTrue);
+    expect(renderEditable.selectPositionAtFrom, const Offset(200.0, 200.0));
+    expect(renderEditable.selectPositionAtTo, const Offset(300.0, 200.0));
+
+    // Move the viewport offset (scroll).
+    renderEditable.offset = ViewportOffset.fixed(150.0);
+    renderEditable.layout(const BoxConstraints.tightFor(width: 400, height: 300.0));
+    await tester.pumpAndSettle();
+
+    await gesture.moveTo(const Offset(300.0, 400.0));
+    await tester.pumpAndSettle();
+    await gesture.up();
+    await tester.pumpAndSettle();
+    expect(renderEditable.selectPositionAtCalled, isTrue);
+    expect(renderEditable.selectPositionAtFrom, const Offset(200.0, 70.0));
+    expect(renderEditable.selectPositionAtTo, const Offset(300.0, 400.0));
+  });
+
   testWidgets('test TextSelectionGestureDetectorBuilder selection disabled', (WidgetTester tester) async {
     await pumpTextSelectionGestureDetectorBuilder(tester, selectionEnabled: false);
     final TestGesture gesture = await tester.startGesture(
@@ -863,9 +901,13 @@ class FakeRenderEditable extends RenderEditable {
   }
 
   bool selectPositionAtCalled = false;
+  Offset? selectPositionAtFrom;
+  Offset? selectPositionAtTo;
   @override
   void selectPositionAt({ required Offset from, Offset? to, required SelectionChangedCause cause }) {
     selectPositionAtCalled = true;
+    selectPositionAtFrom = from;
+    selectPositionAtTo = to;
   }
 
   bool selectWordCalled = false;