Fix text selection when user is dragging in the opposite direction (#29395)

packages/flutter/lib/src/rendering/editable.dart

@@ -1377,10 +1377,18 @@ class RenderEditable extends RenderBox {
       final TextPosition toPosition = to == null
         ? null
         : _textPainter.getPositionForOffset(globalToLocal(to - _paintOffset));
+
+      int baseOffset = fromPosition.offset;
+      int extentOffset = fromPosition.offset;
+      if (toPosition != null) {
+        baseOffset = math.min(fromPosition.offset, toPosition.offset);
+        extentOffset = math.max(fromPosition.offset, toPosition.offset);
+      }
+
       onSelectionChanged(
         TextSelection(
-          baseOffset: fromPosition.offset,
-          extentOffset: toPosition?.offset ?? fromPosition.offset,
+          baseOffset: baseOffset,
+          extentOffset: extentOffset,
           affinity: fromPosition.affinity,
         ),
         this,

packages/flutter/test/material/text_field_test.dart

@@ -645,6 +645,38 @@ void main() {
     expect(controller.selection.extentOffset, testValue.indexOf('g'));
   });
 
+  testWidgets('Dragging in opposite direction also works', (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 ePos = textOffsetToPosition(tester, testValue.indexOf('e'));
+    final Offset gPos = textOffsetToPosition(tester, testValue.indexOf('g'));
+
+    final TestGesture gesture = await tester.startGesture(gPos, kind: PointerDeviceKind.mouse);
+    await tester.pump();
+    await gesture.moveTo(ePos);
+    await tester.pump();
+    await gesture.up();
+    await tester.pumpAndSettle();
+
+    expect(controller.selection.baseOffset, testValue.indexOf('e'));
+    expect(controller.selection.extentOffset, testValue.indexOf('g'));
+  });
+
   testWidgets('Slow mouse dragging also selects text', (WidgetTester tester) async {
     final TextEditingController controller = TextEditingController();
 

packages/flutter/test/rendering/editable_test.dart

@@ -335,4 +335,36 @@ void main() {
     expect(currentSelection.baseOffset, 5);
     expect(currentSelection.extentOffset, 9);
   });
+
+  test('selects correct place when offsets are flipped', () {
+    final TextSelectionDelegate delegate = FakeEditableTextState();
+    final ViewportOffset viewportOffset = ViewportOffset.zero();
+    TextSelection currentSelection;
+    final RenderEditable editable = RenderEditable(
+      backgroundCursorColor: Colors.grey,
+      selectionColor: Colors.black,
+      textDirection: TextDirection.ltr,
+      cursorColor: Colors.red,
+      offset: viewportOffset,
+      textSelectionDelegate: delegate,
+      onSelectionChanged: (TextSelection selection, RenderEditable renderObject, SelectionChangedCause cause) {
+        currentSelection = selection;
+      },
+      text: const TextSpan(
+        text: 'abc def ghi',
+        style: TextStyle(
+          height: 1.0, fontSize: 10.0, fontFamily: 'Ahem',
+        ),
+      ),
+    );
+
+    layout(editable);
+
+    editable.selectPositionAt(from: const Offset(30, 2), to: const Offset(10, 2), cause: SelectionChangedCause.drag);
+    pumpFrame();
+
+    expect(currentSelection.isCollapsed, isFalse);
+    expect(currentSelection.baseOffset, 1);
+    expect(currentSelection.extentOffset, 3);
+  });
 }