Correct text selection pivot points (#71756)

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

@@ -678,6 +678,8 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin {
           newSelection = newSelection.copyWith(extentOffset: textSelection.extentOffset);
         }
       } else {
+        // The directional arrows move the TextSelection.extentOffset, while the
+        // base remains fixed.
         if (rightArrow && newSelection.extentOffset < _plainText.length) {
           int nextExtent;
           if (!shift && !wordModifier && !lineModifier && newSelection.start != newSelection.end) {
@@ -1849,6 +1851,9 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin {
   }
 
   /// Select text between the global positions [from] and [to].
+  ///
+  /// [from] corresponds to the [TextSelection.baseOffset], and [to] corresponds
+  /// to the [TextSelection.extentOffset].
   void selectPositionAt({ required Offset from, Offset? to, required SelectionChangedCause cause }) {
     assert(cause != null);
     assert(from != null);
@@ -1861,12 +1866,8 @@ class RenderEditable extends RenderBox with RelayoutWhenSystemFontsChangeMixin {
       ? 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);
-    }
+    final int baseOffset = fromPosition.offset;
+    final int extentOffset = toPosition?.offset ?? fromPosition.offset;
 
     final TextSelection newSelection = TextSelection(
       baseOffset: baseOffset,

packages/flutter/test/material/text_field_test.dart

@@ -1458,8 +1458,8 @@ void main() {
     await gesture.up();
     await tester.pumpAndSettle();
 
-    expect(controller.selection.baseOffset, testValue.indexOf('e'));
-    expect(controller.selection.extentOffset, testValue.indexOf('g'));
+    expect(controller.selection.baseOffset, testValue.indexOf('g'));
+    expect(controller.selection.extentOffset, testValue.indexOf('e'));
   });
 
   testWidgets('Slow mouse dragging also selects text', (WidgetTester tester) async {

packages/flutter/test/rendering/editable_test.dart

@@ -570,8 +570,8 @@ void main() {
     pumpFrame();
 
     expect(currentSelection.isCollapsed, isFalse);
-    expect(currentSelection.baseOffset, 1);
-    expect(currentSelection.extentOffset, 3);
+    expect(currentSelection.baseOffset, 3);
+    expect(currentSelection.extentOffset, 1);
   });
 
   test('selection does not flicker as user is dragging', () {
@@ -1006,7 +1006,79 @@ void main() {
     await simulateKeyUpEvent(LogicalKeyboardKey.arrowLeft);
     expect(currentSelection.isCollapsed, true);
     expect(currentSelection.baseOffset, 2);
+  }, skip: isBrowser); // https://github.com/flutter/flutter/issues/58068
 
+  test('arrow keys with selection text and shift', () async {
+    final TextSelectionDelegate delegate = FakeEditableTextState();
+    final ViewportOffset viewportOffset = ViewportOffset.zero();
+    late 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;
+      },
+      startHandleLayerLink: LayerLink(),
+      endHandleLayerLink: LayerLink(),
+      text: const TextSpan(
+        text: '012345',  // Thumbs up
+        style: TextStyle(height: 1.0, fontSize: 10.0, fontFamily: 'Ahem'),
+      ),
+      selection: const TextSelection.collapsed(
+        offset: 0,
+      ),
+    );
+
+    layout(editable);
+    editable.hasFocus = true;
+
+    editable.selection = const TextSelection(baseOffset: 2, extentOffset: 4);
+    pumpFrame();
+
+    await simulateKeyDownEvent(LogicalKeyboardKey.shift);
+    await simulateKeyDownEvent(LogicalKeyboardKey.arrowRight);
+    await simulateKeyUpEvent(LogicalKeyboardKey.arrowRight);
+    await simulateKeyUpEvent(LogicalKeyboardKey.shift);
+    expect(currentSelection.isCollapsed, false);
+    expect(currentSelection.baseOffset, 2);
+    expect(currentSelection.extentOffset, 5);
+
+    editable.selection = const TextSelection(baseOffset: 4, extentOffset: 2);
+    pumpFrame();
+
+    await simulateKeyDownEvent(LogicalKeyboardKey.shift);
+    await simulateKeyDownEvent(LogicalKeyboardKey.arrowRight);
+    await simulateKeyUpEvent(LogicalKeyboardKey.arrowRight);
+    await simulateKeyUpEvent(LogicalKeyboardKey.shift);
+    expect(currentSelection.isCollapsed, false);
+    expect(currentSelection.baseOffset, 4);
+    expect(currentSelection.extentOffset, 3);
+
+    editable.selection = const TextSelection(baseOffset: 2, extentOffset: 4);
+    pumpFrame();
+
+    await simulateKeyDownEvent(LogicalKeyboardKey.shift);
+    await simulateKeyDownEvent(LogicalKeyboardKey.arrowLeft);
+    await simulateKeyUpEvent(LogicalKeyboardKey.arrowLeft);
+    await simulateKeyUpEvent(LogicalKeyboardKey.shift);
+    expect(currentSelection.isCollapsed, false);
+    expect(currentSelection.baseOffset, 2);
+    expect(currentSelection.extentOffset, 3);
+
+    editable.selection = const TextSelection(baseOffset: 4, extentOffset: 2);
+    pumpFrame();
+
+    await simulateKeyDownEvent(LogicalKeyboardKey.shift);
+    await simulateKeyDownEvent(LogicalKeyboardKey.arrowLeft);
+    await simulateKeyUpEvent(LogicalKeyboardKey.arrowLeft);
+    await simulateKeyUpEvent(LogicalKeyboardKey.shift);
+    expect(currentSelection.isCollapsed, false);
+    expect(currentSelection.baseOffset, 4);
+    expect(currentSelection.extentOffset, 1);
   }, skip: isBrowser); // https://github.com/flutter/flutter/issues/58068
 
   group('delete', () {

packages/flutter/test/widgets/selectable_text_test.dart

@@ -765,8 +765,8 @@ void main() {
     await gesture.up();
     await tester.pumpAndSettle();
 
-    expect(controller.selection.baseOffset, 5);
-    expect(controller.selection.extentOffset, 8);
+    expect(controller.selection.baseOffset, 8);
+    expect(controller.selection.extentOffset, 5);
   });
 
   testWidgets('Slow mouse dragging also selects text', (WidgetTester tester) async {