Fix iOS touch drag behavior (#125169)

Before this change on a quick touch drag the cursor, where the touch is not on the previous collapsed selection, the cursor would move to the tapped position.

After this change on a quick touch drag the cursor does not move unless the touch is on the previously collapsed selection. This is inline with native behavior.

Before|After|Native
--|--|--
<video src="https://user-images.githubusercontent.com/948037/233224775-f33b42b5-5638-416c-9278-39ecd964e3bb.mov" />|<video src="https://user-images.githubusercontent.com/948037/233224760-2d1af657-8d99-45fc-8499-9567f17d533e.mov" />|<video src="https://user-images.githubusercontent.com/948037/233224790-f5997cfa-7370-4891-8952-11ef8057a729.mov" />

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

@@ -2587,6 +2587,7 @@ class TextSelectionGestureDetectorBuilder {
     _dragStartSelection = renderEditable.selection;
     _dragStartScrollOffset = _scrollPosition;
     _dragStartViewportOffset = renderEditable.offset.pixels;
+    _dragBeganOnPreviousSelection = _positionOnSelection(details.globalPosition, _dragStartSelection);
 
     if (_TextSelectionGestureDetectorState._getEffectiveConsecutiveTapCount(details.consecutiveTapCount) > 1) {
       // Do not set the selection on a consecutive tap and drag.
@@ -2609,6 +2610,29 @@ class TextSelectionGestureDetectorBuilder {
     } else {
       switch (defaultTargetPlatform) {
         case TargetPlatform.iOS:
+          switch (details.kind) {
+            case PointerDeviceKind.mouse:
+            case PointerDeviceKind.trackpad:
+              renderEditable.selectPositionAt(
+                from: details.globalPosition,
+                cause: SelectionChangedCause.drag,
+              );
+            case PointerDeviceKind.stylus:
+            case PointerDeviceKind.invertedStylus:
+            case PointerDeviceKind.touch:
+            case PointerDeviceKind.unknown:
+              // For iOS platforms, a touch drag does not initiate unless the
+              // editable has focus and the drag began on the previous selection.
+              assert(_dragBeganOnPreviousSelection != null);
+              if (renderEditable.hasFocus && _dragBeganOnPreviousSelection!) {
+                renderEditable.selectPositionAt(
+                  from: details.globalPosition,
+                  cause: SelectionChangedCause.drag,
+                );
+                _showMagnifierIfSupportedByPlatform(details.globalPosition);
+              }
+            case null:
+          }
         case TargetPlatform.android:
         case TargetPlatform.fuchsia:
           switch (details.kind) {
@@ -2632,7 +2656,6 @@ class TextSelectionGestureDetectorBuilder {
                 _showMagnifierIfSupportedByPlatform(details.globalPosition);
               }
             case null:
-              break;
           }
         case TargetPlatform.linux:
         case TargetPlatform.macOS:
@@ -2754,7 +2777,6 @@ class TextSelectionGestureDetectorBuilder {
             case PointerDeviceKind.invertedStylus:
             case PointerDeviceKind.touch:
             case PointerDeviceKind.unknown:
-              _dragBeganOnPreviousSelection ??= _positionOnSelection(dragStartGlobalPosition, _dragStartSelection);
               assert(_dragBeganOnPreviousSelection != null);
               if (renderEditable.hasFocus
                   && _dragStartSelection!.isCollapsed

packages/flutter/test/cupertino/text_field_test.dart

@@ -5372,6 +5372,51 @@ void main() {
     expect(controller.selection.extentOffset, testValue.indexOf('g'));
   });
 
+  testWidgets('Cursor should not move on a quick touch drag when touch does not begin on previous selection (iOS)', (WidgetTester tester) async {
+    final TextEditingController controller = TextEditingController();
+
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: CupertinoPageScaffold(
+          child: CupertinoTextField(
+            dragStartBehavior: DragStartBehavior.down,
+            controller: controller,
+          ),
+        ),
+      ),
+    );
+
+    const String testValue = 'abc def ghi';
+    await tester.enterText(find.byType(CupertinoTextField), testValue);
+    await tester.pumpAndSettle(const Duration(milliseconds: 200));
+
+    final Offset aPos = textOffsetToPosition(tester, testValue.indexOf('a'));
+    final Offset iPos = textOffsetToPosition(tester, testValue.indexOf('i'));
+
+    // Tap on text field to gain focus, and set selection to '|a'. On iOS
+    // the selection is set to the word edge closest to the tap position.
+    // We await for [kDoubleTapTimeout] after the up event, so our next down
+    // event does not register as a double tap.
+    final TestGesture gesture = await tester.startGesture(aPos);
+    await tester.pump();
+    await gesture.up();
+    await tester.pumpAndSettle(kDoubleTapTimeout);
+
+    expect(controller.selection.isCollapsed, true);
+    expect(controller.selection.baseOffset, 0);
+
+    // The position we tap during a drag start is not on the collapsed selection,
+    // so the cursor should not move.
+    await gesture.down(textOffsetToPosition(tester, 7));
+    await gesture.moveTo(iPos);
+    await tester.pumpAndSettle();
+
+    expect(controller.selection.isCollapsed, true);
+    expect(controller.selection.baseOffset, 0);
+  },
+    variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.iOS }),
+  );
+
   testWidgets('Can move cursor when dragging, when tap is on collapsed selection (iOS)', (WidgetTester tester) async {
     final TextEditingController controller = TextEditingController();
 

packages/flutter/test/material/text_field_test.dart

@@ -2189,6 +2189,51 @@ void main() {
     variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.iOS }),
   );
 
+  testWidgets('Cursor should not move on a quick touch drag when touch does not begin on previous selection (iOS)', (WidgetTester tester) async {
+    final TextEditingController controller = TextEditingController();
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Material(
+          child: TextField(
+            dragStartBehavior: DragStartBehavior.down,
+            controller: controller,
+          ),
+        ),
+      ),
+    );
+
+    const String testValue = 'abc def ghi';
+    await tester.enterText(find.byType(TextField), testValue);
+    await skipPastScrollingAnimation(tester);
+
+    final Offset aPos = textOffsetToPosition(tester, testValue.indexOf('a'));
+    final Offset iPos = textOffsetToPosition(tester, testValue.indexOf('i'));
+
+    // Tap on text field to gain focus, and set selection to '|a'. On iOS
+    // the selection is set to the word edge closest to the tap position.
+    // We await for kDoubleTapTimeout after the up event, so our next down event
+    // does not register as a double tap.
+    final TestGesture gesture = await tester.startGesture(aPos);
+    await tester.pump();
+    await gesture.up();
+    await tester.pumpAndSettle(kDoubleTapTimeout);
+
+    expect(controller.selection.isCollapsed, true);
+    expect(controller.selection.baseOffset, 0);
+
+    // The position we tap during a drag start is not on the collapsed selection,
+    // so the cursor should not move.
+    await gesture.down(textOffsetToPosition(tester, 7));
+    await gesture.moveTo(iPos);
+    await tester.pumpAndSettle();
+
+    expect(controller.selection.isCollapsed, true);
+    expect(controller.selection.baseOffset, 0);
+  },
+    variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.iOS }),
+  );
+
   testWidgets('Can move cursor when dragging, when tap is on collapsed selection (iOS) - multiline', (WidgetTester tester) async {
     final TextEditingController controller = TextEditingController();