Implement SelectionArea single click/tap gestures (#132682)

This change collapses the selection at the clicked/tapped location on single click down for desktop platforms, and on single click/tap up for mobile platforms to match native.

This is a change from how `SelectionArea` previously worked. Before this change a single click down would clear the selection. From observing a native browser it looks like when tapping on static text the selection is not cleared but collapsed. A user can still attain the selection from static text using the `window.getSelection` API.

https://jsfiddle.net/juepasn3/11/ You can try this demo out here to observe this behavior yourself. When clicking on static text the selection will change.

This change also allows `Paragraph.selections` to return selections that are collapsed. This for testing purposes to confirm where the selection has been collapsed.

Partially fixes: #129583

examples/api/test/material/context_menu/selectable_region_toolbar_builder.0_test.dart

@@ -24,6 +24,9 @@ void main() {
 
     // Right clicking the Text in the SelectionArea shows the custom context
     // menu.
+    final TestGesture primaryMouseButtonGesture = await tester.createGesture(
+      kind: PointerDeviceKind.mouse,
+    );
     final TestGesture gesture = await tester.startGesture(
       tester.getCenter(find.text(example.text)),
       kind: PointerDeviceKind.mouse,
@@ -37,7 +40,9 @@ void main() {
     expect(find.text('Print'), findsOneWidget);
 
     // Tap to dismiss.
-    await tester.tapAt(tester.getCenter(find.byType(Scaffold)));
+    await primaryMouseButtonGesture.down(tester.getCenter(find.byType(Scaffold)));
+    await tester.pump();
+    await primaryMouseButtonGesture.up();
     await tester.pumpAndSettle();
 
     expect(find.byType(AdaptiveTextSelectionToolbar), findsNothing);

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

@@ -132,7 +132,7 @@ mixin RenderInlineChildrenContainerDefaults on RenderBox, ContainerRenderObjectM
             ui.PlaceholderAlignment.belowBaseline ||
             ui.PlaceholderAlignment.bottom ||
             ui.PlaceholderAlignment.middle ||
-            ui.PlaceholderAlignment.top      => null,
+            ui.PlaceholderAlignment.top => null,
             ui.PlaceholderAlignment.baseline => child.getDistanceToBaseline(span.baseline!),
           },
         );
@@ -351,8 +351,7 @@ class RenderParagraph extends RenderBox with ContainerRenderObjectMixin<RenderBo
     final List<TextSelection> results = <TextSelection>[];
     for (final _SelectableFragment fragment in _lastSelectableFragments!) {
       if (fragment._textSelectionStart != null &&
-          fragment._textSelectionEnd != null &&
-          fragment._textSelectionStart!.offset != fragment._textSelectionEnd!.offset) {
+          fragment._textSelectionEnd != null) {
         results.add(
           TextSelection(
             baseOffset: fragment._textSelectionStart!.offset,
@@ -1309,9 +1308,9 @@ class RenderParagraph extends RenderBox with ContainerRenderObjectMixin<RenderBo
 
 /// A continuous, selectable piece of paragraph.
 ///
-/// Since the selections in [PlaceHolderSpan] are handled independently in its
+/// Since the selections in [PlaceholderSpan] are handled independently in its
 /// subtree, a selection in [RenderParagraph] can't continue across a
-/// [PlaceHolderSpan]. The [RenderParagraph] splits itself on [PlaceHolderSpan]
+/// [PlaceholderSpan]. The [RenderParagraph] splits itself on [PlaceholderSpan]
 /// to create multiple `_SelectableFragment`s so that they can be selected
 /// separately.
 class _SelectableFragment with Selectable, ChangeNotifier implements TextLayoutMetrics {
@@ -1720,7 +1719,7 @@ class _SelectableFragment with Selectable, ChangeNotifier implements TextLayoutM
     _selectableContainsOriginWord = true;
 
     final TextPosition position = paragraph.getPositionForOffset(paragraph.globalToLocal(globalPosition));
-    if (_positionIsWithinCurrentSelection(position)) {
+    if (_positionIsWithinCurrentSelection(position) && _textSelectionStart != _textSelectionEnd) {
       return SelectionResult.end;
     }
     final _WordBoundaryRecord wordBoundary = _getWordBoundaryAtPosition(position);

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

@@ -670,7 +670,7 @@ class ScrollableState extends State<Scrollable> with TickerProviderStateMixin, R
       if (oldWidget.controller == null) {
         // The old controller was null, meaning the fallback cannot be null.
         // Dispose of the fallback.
-        assert(_fallbackScrollController !=  null);
+        assert(_fallbackScrollController != null);
         assert(widget.controller != null);
         _fallbackScrollController!.detach(position);
         _fallbackScrollController!.dispose();
@@ -1954,7 +1954,7 @@ class TwoDimensionalScrollableState extends State<TwoDimensionalScrollable> {
       if (oldWidget.horizontalDetails.controller == null) {
         // The old controller was null, meaning the fallback cannot be null.
         // Dispose of the fallback.
-        assert(_horizontalFallbackController !=  null);
+        assert(_horizontalFallbackController != null);
         assert(widget.horizontalDetails.controller != null);
         _horizontalFallbackController!.dispose();
         _horizontalFallbackController = null;

packages/flutter/test/material/selection_area_test.dart

@@ -224,8 +224,14 @@ void main() {
 
     // Backwards selection.
     await gesture.down(textOffsetToPosition(paragraph, 3));
-    await tester.pumpAndSettle();
-    expect(content, isNull);
+    await tester.pump();
+    await gesture.up();
+    await tester.pumpAndSettle(kDoubleTapTimeout);
+    expect(content, isNotNull);
+    expect(content!.plainText, '');
+
+    await gesture.down(textOffsetToPosition(paragraph, 3));
+    await tester.pump();
     await gesture.moveTo(textOffsetToPosition(paragraph, 0));
     await gesture.up();
     await tester.pump();

packages/flutter/test/rendering/paragraph_test.dart

@@ -978,7 +978,8 @@ void main() {
           granularity: TextGranularity.word,
         ),
       );
-      expect(paragraph.selections.length, 0); // how []are you
+      expect(paragraph.selections.length, 1); // how []are you
+      expect(paragraph.selections[0], const TextSelection.collapsed(offset: 4));
 
       // Equivalent to sending shift + alt + arrow-left.
       registrar.selectables[0].dispatchSelectionEvent(