Text handle drag swap on Apple platforms (#105069)

Dragging the base text selection handle on Apple makes it the extent.

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

@@ -459,13 +459,31 @@ class TextSelectionOverlay {
       return;
     }
 
-    final TextSelection newSelection = TextSelection(
-      baseOffset: _selection.baseOffset,
-      extentOffset: position.offset,
-    );
-
-    if (newSelection.baseOffset >= newSelection.extentOffset) {
-      return; // Don't allow order swapping.
+    final TextSelection newSelection;
+    switch (defaultTargetPlatform) {
+      // On Apple platforms, dragging the base handle makes it the extent.
+      case TargetPlatform.iOS:
+      case TargetPlatform.macOS:
+        newSelection = TextSelection(
+          extentOffset: position.offset,
+          baseOffset: _selection.start,
+        );
+        if (position.offset <= _selection.start) {
+          return; // Don't allow order swapping.
+        }
+        break;
+      case TargetPlatform.android:
+      case TargetPlatform.fuchsia:
+      case TargetPlatform.linux:
+      case TargetPlatform.windows:
+        newSelection = TextSelection(
+          baseOffset: _selection.baseOffset,
+          extentOffset: position.offset,
+        );
+        if (newSelection.baseOffset >= newSelection.extentOffset) {
+          return; // Don't allow order swapping.
+        }
+        break;
     }
 
     _handleSelectionHandleChanged(newSelection, isEnd: true);
@@ -489,13 +507,31 @@ class TextSelectionOverlay {
       return;
     }
 
-    final TextSelection newSelection = TextSelection(
-      baseOffset: position.offset,
-      extentOffset: _selection.extentOffset,
-    );
-
-    if (newSelection.baseOffset >= newSelection.extentOffset) {
-      return; // Don't allow order swapping.
+    final TextSelection newSelection;
+    switch (defaultTargetPlatform) {
+      // On Apple platforms, dragging the base handle makes it the extent.
+      case TargetPlatform.iOS:
+      case TargetPlatform.macOS:
+        newSelection = TextSelection(
+          extentOffset: position.offset,
+          baseOffset: _selection.end,
+        );
+        if (newSelection.extentOffset >= _selection.end) {
+          return; // Don't allow order swapping.
+        }
+        break;
+      case TargetPlatform.android:
+      case TargetPlatform.fuchsia:
+      case TargetPlatform.linux:
+      case TargetPlatform.windows:
+        newSelection = TextSelection(
+          baseOffset: position.offset,
+          extentOffset: _selection.extentOffset,
+        );
+        if (newSelection.baseOffset >= newSelection.extentOffset) {
+          return; // Don't allow order swapping.
+        }
+        break;
     }
 
     _handleSelectionHandleChanged(newSelection, isEnd: false);
@@ -1116,7 +1152,6 @@ class _SelectionHandleOverlay extends StatefulWidget {
 }
 
 class _SelectionHandleOverlayState extends State<_SelectionHandleOverlay> with SingleTickerProviderStateMixin {
-
   late AnimationController _controller;
   Animation<double> get _opacity => _controller.view;
 

packages/flutter/test/material/text_field_test.dart

@@ -2077,7 +2077,7 @@ void main() {
     expect(selection.extentOffset, 7);
 
     final RenderEditable renderEditable = findRenderEditable(tester);
-    final List<TextSelectionPoint> endpoints = globalize(
+    List<TextSelectionPoint> endpoints = globalize(
       renderEditable.getEndpointsForSelection(selection),
       renderEditable,
     );
@@ -2100,6 +2100,36 @@ void main() {
 
     // Drag the left handle 2 letters to the left.
     handlePos = endpoints[0].point + const Offset(-1.0, 1.0);
+    newHandlePos = textOffsetToPosition(tester, 2);
+    gesture = await tester.startGesture(handlePos, pointer: 7);
+    await tester.pump();
+    await gesture.moveTo(newHandlePos);
+    await tester.pump();
+    await gesture.up();
+    await tester.pump();
+
+    switch (defaultTargetPlatform) {
+      // On Apple platforms, dragging the base handle makes it the extent.
+      case TargetPlatform.iOS:
+      case TargetPlatform.macOS:
+        expect(controller.selection.baseOffset, 11);
+        expect(controller.selection.extentOffset, 2);
+        break;
+      case TargetPlatform.android:
+      case TargetPlatform.fuchsia:
+      case TargetPlatform.linux:
+      case TargetPlatform.windows:
+        expect(controller.selection.baseOffset, 2);
+        expect(controller.selection.extentOffset, 11);
+        break;
+    }
+
+    // Drag the left handle 2 letters to the left again.
+    endpoints = globalize(
+      renderEditable.getEndpointsForSelection(controller.selection),
+      renderEditable,
+    );
+    handlePos = endpoints[0].point + const Offset(-1.0, 1.0);
     newHandlePos = textOffsetToPosition(tester, 0);
     gesture = await tester.startGesture(handlePos, pointer: 7);
     await tester.pump();
@@ -2108,9 +2138,24 @@ void main() {
     await gesture.up();
     await tester.pump();
 
-    expect(controller.selection.baseOffset, 0);
-    expect(controller.selection.extentOffset, 11);
-  });
+    switch (defaultTargetPlatform) {
+      case TargetPlatform.iOS:
+      case TargetPlatform.macOS:
+        // The left handle was already the extent, and it remains so.
+        expect(controller.selection.baseOffset, 11);
+        expect(controller.selection.extentOffset, 0);
+        break;
+      case TargetPlatform.android:
+      case TargetPlatform.fuchsia:
+      case TargetPlatform.linux:
+      case TargetPlatform.windows:
+        expect(controller.selection.baseOffset, 0);
+        expect(controller.selection.extentOffset, 11);
+        break;
+    }
+  },
+    variant: TargetPlatformVariant.all(),
+  );
 
   testWidgets('Cannot drag one handle past the other', (WidgetTester tester) async {
     final TextEditingController controller = TextEditingController();