Add a threshold when comparing screen order for selectables. (#130043)

Add a threshold when comparing screen order for selectables. So when the vertical position diff is within the threshold, will compare the horizontal position.

This fixes https://github.com/flutter/flutter/issues/111021 and https://github.com/flutter/flutter/issues/127942

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

@@ -39,6 +39,11 @@ const Set<PointerDeviceKind> _kLongPressSelectionDevices = <PointerDeviceKind>{
   PointerDeviceKind.invertedStylus,
 };
 
+// In practice some selectables like widgetspan shift several pixels. So when
+// the vertical position diff is within the threshold, compare the horizontal
+// position to make the compareScreenOrder function more robust.
+const double _kSelectableVerticalComparingThreshold = 3.0;
+
 /// A widget that introduces an area for user selections.
 ///
 /// Flutter widgets are not selectable by default. Wrapping a widget subtree
@@ -1703,11 +1708,11 @@ abstract class MultiSelectableSelectionContainerDelegate extends SelectionContai
   /// Returns positive if a is lower, negative if a is higher, 0 if their
   /// order can't be determine solely by their vertical position.
   static int _compareVertically(Rect a, Rect b) {
-    if ((a.top - b.top < precisionErrorTolerance && a.bottom - b.bottom > - precisionErrorTolerance) ||
-        (b.top - a.top < precisionErrorTolerance && b.bottom - a.bottom > - precisionErrorTolerance)) {
+    if ((a.top - b.top < _kSelectableVerticalComparingThreshold && a.bottom - b.bottom > - _kSelectableVerticalComparingThreshold) ||
+        (b.top - a.top < _kSelectableVerticalComparingThreshold && b.bottom - a.bottom > - _kSelectableVerticalComparingThreshold)) {
       return 0;
     }
-    if ((a.top - b.top).abs() > precisionErrorTolerance) {
+    if ((a.top - b.top).abs() > _kSelectableVerticalComparingThreshold) {
       return a.top > b.top ? 1 : -1;
     }
     return a.bottom > b.bottom ? 1 : -1;

packages/flutter/test/widgets/selectable_region_test.dart

@@ -2469,6 +2469,51 @@ void main() {
       skip: !kIsWeb, // [intended]
     );
   });
+
+  testWidgets('Multiple selectables on a single line should be in screen order', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/127942.
+    final UniqueKey outerText = UniqueKey();
+    const TextStyle textStyle = TextStyle(fontSize: 10);
+    await tester.pumpWidget(
+      MaterialApp(
+        home: SelectableRegion(
+          focusNode: FocusNode(),
+          selectionControls: materialTextSelectionControls,
+          child: Scaffold(
+            body: Center(
+              child: Text.rich(
+                const TextSpan(
+                  children: <InlineSpan>[
+                    TextSpan(text: 'Hello my name is ', style: textStyle),
+                    WidgetSpan(
+                      child: Text('Dash', style: textStyle),
+                      alignment: PlaceholderAlignment.middle,
+                    ),
+                    TextSpan(text: '.', style: textStyle),
+                  ],
+                ),
+                key: outerText,
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+    final RenderParagraph paragraph1 = tester.renderObject<RenderParagraph>(find.descendant(of: find.byKey(outerText), matching: find.byType(RichText)).first);
+    final TestGesture gesture = await tester.startGesture(textOffsetToPosition(paragraph1, 0), kind: PointerDeviceKind.mouse);
+    addTearDown(gesture.removePointer);
+    await tester.pump();
+    await gesture.up();
+
+    // Select all.
+    await sendKeyCombination(tester, const SingleActivator(LogicalKeyboardKey.keyA, control: true));
+
+    // keyboard copy.
+    await sendKeyCombination(tester, const SingleActivator(LogicalKeyboardKey.keyC, control: true));
+
+    final Map<String, dynamic> clipboardData = mockClipboard.clipboardData as Map<String, dynamic>;
+    expect(clipboardData['text'], 'Hello my name is Dash.');
+  });
 }
 
 class SelectionSpy extends LeafRenderObjectWidget {