Fix text selection handles showing outside the visible text region (#24476)

Don't show handles outside the text field's boundary.

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

@@ -255,6 +255,50 @@ class RenderEditable extends RenderBox {
 
   Rect _lastCaretRect;
 
+  /// Track whether position of the start of the selected text is within the viewport.
+  ///
+  /// For example, if the text contains "Hello World", and the user selects
+  /// "Hello", then scrolls so only "World" is visible, this will become false.
+  /// If the user scrolls back so that the "H" is visible again, this will
+  /// become true.
+  ///
+  /// This bool indicates whether the text is scrolled so that the handle is
+  /// inside the text field viewport, as opposed to whether it is actually
+  /// visible on the screen.
+  ValueListenable<bool> get selectionStartInViewport => _selectionStartInViewport;
+  final ValueNotifier<bool> _selectionStartInViewport = ValueNotifier<bool>(true);
+
+  /// Track whether position of the end of the selected text is within the viewport.
+  ///
+  /// For example, if the text contains "Hello World", and the user selects
+  /// "World", then scrolls so only "Hello" is visible, this will become
+  /// 'false'. If the user scrolls back so that the "d" is visible again, this
+  /// will become 'true'.
+  ///
+  /// This bool indicates whether the text is scrolled so that the handle is
+  /// inside the text field viewport, as opposed to whether it is actually
+  /// visible on the screen.
+  ValueListenable<bool> get selectionEndInViewport => _selectionEndInViewport;
+  final ValueNotifier<bool> _selectionEndInViewport = ValueNotifier<bool>(true);
+
+  void _updateSelectionExtentsVisibility(Offset effectiveOffset) {
+    final Rect visibleRegion = Offset.zero & size;
+
+    final Offset startOffset = _textPainter.getOffsetForCaret(
+      TextPosition(offset: _selection.start, affinity: _selection.affinity),
+      Rect.zero
+    );
+
+    _selectionStartInViewport.value = visibleRegion.contains(startOffset + effectiveOffset);
+
+    final Offset endOffset =  _textPainter.getOffsetForCaret(
+      TextPosition(offset: _selection.end, affinity: _selection.affinity),
+      Rect.zero
+    );
+
+    _selectionEndInViewport.value = visibleRegion.contains(endOffset + effectiveOffset);
+  }
+
   static const int _kLeftArrowCode = 21;
   static const int _kRightArrowCode = 22;
   static const int _kUpArrowCode = 19;
@@ -1570,6 +1614,7 @@ class RenderEditable extends RenderBox {
         showCaret = true;
       else if (!_selection.isCollapsed && _selectionColor != null)
         showSelection = true;
+      _updateSelectionExtentsVisibility(effectiveOffset);
     }
 
     if (showSelection) {

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

@@ -4,11 +4,12 @@
 
 import 'dart:async';
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart' show kDoubleTapTimeout, kDoubleTapSlop;
+import 'package:flutter/gestures.dart';
 import 'package:flutter/rendering.dart';
-import 'package:flutter/services.dart';
 import 'package:flutter/scheduler.dart';
-import 'package:flutter/gestures.dart';
+import 'package:flutter/services.dart';
 
 import 'basic.dart';
 import 'container.dart';
@@ -16,6 +17,7 @@ import 'editable_text.dart';
 import 'framework.dart';
 import 'gesture_detector.dart';
 import 'overlay.dart';
+import 'ticker_provider.dart';
 import 'transitions.dart';
 
 export 'package:flutter/services.dart' show TextSelectionDelegate;
@@ -253,8 +255,7 @@ class TextSelectionOverlay {
        _value = value {
     final OverlayState overlay = Overlay.of(context);
     assert(overlay != null);
-    _handleController = AnimationController(duration: _fadeDuration, vsync: overlay);
-    _toolbarController = AnimationController(duration: _fadeDuration, vsync: overlay);
+    _toolbarController = AnimationController(duration: fadeDuration, vsync: overlay);
   }
 
   /// The context in which the selection handles should appear.
@@ -299,11 +300,10 @@ class TextSelectionOverlay {
   ///  * [DragGestureRecognizer.dragStartBehavior], which gives an example for the different behaviors.
   final DragStartBehavior dragStartBehavior;
 
-  /// Controls the fade-in animations.
-  static const Duration _fadeDuration = Duration(milliseconds: 150);
-  AnimationController _handleController;
+  /// Controls the fade-in and fade-out animations for the toolbar and handles.
+  static const Duration fadeDuration = Duration(milliseconds: 150);
+
   AnimationController _toolbarController;
-  Animation<double> get _handleOpacity => _handleController.view;
   Animation<double> get _toolbarOpacity => _toolbarController.view;
 
   TextEditingValue _value;
@@ -325,7 +325,6 @@ class TextSelectionOverlay {
       OverlayEntry(builder: (BuildContext context) => _buildHandle(context, _TextSelectionHandlePosition.end)),
     ];
     Overlay.of(context, debugRequiredFor: debugRequiredFor).insertAll(_handles);
-    _handleController.forward(from: 0.0);
   }
 
   /// Shows the toolbar by inserting it into the [context]'s overlay.
@@ -388,14 +387,12 @@ class TextSelectionOverlay {
     _toolbar?.remove();
     _toolbar = null;
 
-    _handleController.stop();
     _toolbarController.stop();
   }
 
   /// Final cleanup.
   void dispose() {
     hide();
-    _handleController.dispose();
     _toolbarController.dispose();
   }
 
@@ -403,9 +400,7 @@ class TextSelectionOverlay {
     if ((_selection.isCollapsed && position == _TextSelectionHandlePosition.end) ||
          selectionControls == null)
       return Container(); // hide the second handle when collapsed
-    return FadeTransition(
-      opacity: _handleOpacity,
-      child: _TextSelectionHandleOverlay(
+    return _TextSelectionHandleOverlay(
       onSelectionHandleChanged: (TextSelection newSelection) { _handleSelectionHandleChanged(newSelection, position); },
       onSelectionHandleTapped: _handleSelectionHandleTapped,
       layerLink: layerLink,
@@ -414,7 +409,6 @@ class TextSelectionOverlay {
       selectionControls: selectionControls,
       position: position,
       dragStartBehavior: dragStartBehavior,
-      ),
     );
   }
 
@@ -498,11 +492,58 @@ class _TextSelectionHandleOverlay extends StatefulWidget {
 
   @override
   _TextSelectionHandleOverlayState createState() => _TextSelectionHandleOverlayState();
+
+  ValueListenable<bool> get _visibility {
+    switch (position) {
+      case _TextSelectionHandlePosition.start:
+        return renderObject.selectionStartInViewport;
+      case _TextSelectionHandlePosition.end:
+        return renderObject.selectionEndInViewport;
+    }
+    return null;
+  }
 }
 
-class _TextSelectionHandleOverlayState extends State<_TextSelectionHandleOverlay> {
+class _TextSelectionHandleOverlayState
+    extends State<_TextSelectionHandleOverlay> with SingleTickerProviderStateMixin {
   Offset _dragPosition;
 
+  AnimationController _controller;
+  Animation<double> get _opacity => _controller.view;
+
+  @override
+  void initState() {
+    super.initState();
+
+    _controller = AnimationController(duration: TextSelectionOverlay.fadeDuration, vsync: this);
+
+    _handleVisibilityChanged();
+    widget._visibility.addListener(_handleVisibilityChanged);
+  }
+
+  void _handleVisibilityChanged() {
+    if (widget._visibility.value) {
+      _controller.forward();
+    } else {
+      _controller.reverse();
+    }
+  }
+
+  @override
+  void didUpdateWidget(_TextSelectionHandleOverlay oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    oldWidget._visibility.removeListener(_handleVisibilityChanged);
+    _handleVisibilityChanged();
+    widget._visibility.addListener(_handleVisibilityChanged);
+  }
+
+  @override
+  void dispose() {
+    widget._visibility.removeListener(_handleVisibilityChanged);
+    _controller.dispose();
+    super.dispose();
+  }
+
   void _handleDragStart(DragStartDetails details) {
     _dragPosition = details.globalPosition + Offset(0.0, -widget.selectionControls.handleSize.height);
   }
@@ -562,9 +603,18 @@ class _TextSelectionHandleOverlayState extends State<_TextSelectionHandleOverlay
         break;
     }
 
+    final Size viewport = widget.renderObject.size;
+    point = Offset(
+      point.dx.clamp(0.0, viewport.width),
+      point.dy.clamp(0.0, viewport.height),
+    );
+
+
     return CompositedTransformFollower(
       link: widget.layerLink,
       showWhenUnlinked: false,
+      child: FadeTransition(
+        opacity: _opacity,
         child: GestureDetector(
           dragStartBehavior: widget.dragStartBehavior,
           onPanStart: _handleDragStart,
@@ -587,6 +637,7 @@ class _TextSelectionHandleOverlayState extends State<_TextSelectionHandleOverlay
             ],
           ),
         ),
+      )
     );
   }
 

packages/flutter/test/widgets/editable_text_test.dart

@@ -1941,6 +1941,145 @@ void main() {
     expect(renderEditable.text.text, 'text composing text');
     expect(renderEditable.text.style.decoration, isNull);
   });
+
+  testWidgets('text selection handle visibility', (WidgetTester tester) async {
+    final GlobalKey<EditableTextState> editableTextKey =
+        GlobalKey<EditableTextState>();
+
+    const String testText = 'XXXXX          XXXXX';
+    final TextEditingController controller = TextEditingController(text: testText);
+
+    final Widget widget = MaterialApp(
+      home: Align(
+        alignment: Alignment.topLeft,
+        child: SizedBox(
+          width: 100,
+          child: EditableText(
+            key: editableTextKey,
+            controller: controller,
+            focusNode: FocusNode(),
+            style: Typography(platform: TargetPlatform.android).black.subhead,
+            cursorColor: Colors.blue,
+            backgroundCursorColor: Colors.grey,
+            selectionControls: materialTextSelectionControls,
+            keyboardType: TextInputType.text,
+          ),
+        ),
+      ),
+    );
+
+    await tester.pumpWidget(widget);
+
+    final EditableTextState state =
+        tester.state<EditableTextState>(find.byType(EditableText));
+    final RenderEditable renderEditable = state.renderEditable;
+    final Scrollable scrollable = tester.widget<Scrollable>(find.byType(Scrollable));
+
+    bool leftVisibleBefore = false;
+    bool rightVisibleBefore = false;
+
+    Future<void> verifyVisibility(
+      bool leftVisible,
+      Symbol leftPosition,
+      bool rightVisible,
+      Symbol rightPosition,
+    ) async {
+      await tester.pump();
+
+      // Check the signal from RenderEditable about whether they're within the
+      // viewport.
+
+      expect(renderEditable.selectionStartInViewport.value, equals(leftVisible));
+      expect(renderEditable.selectionEndInViewport.value, equals(rightVisible));
+
+      // Check that the animations are functional and going in the right
+      // direction.
+
+      final List<Widget> transitions =
+        find.byType(FadeTransition).evaluate().map((Element e) => e.widget).toList();
+      final FadeTransition left = transitions[1];
+      final FadeTransition right = transitions[2];
+
+      if (leftVisibleBefore)
+        expect(left.opacity.value, equals(1.0));
+      if (rightVisibleBefore)
+        expect(right.opacity.value, equals(1.0));
+
+      await tester.pump(TextSelectionOverlay.fadeDuration ~/ 2);
+
+      if (leftVisible != leftVisibleBefore)
+        expect(left.opacity.value, equals(0.5));
+      if (rightVisible != rightVisibleBefore)
+        expect(right.opacity.value, equals(0.5));
+
+      await tester.pump(TextSelectionOverlay.fadeDuration ~/ 2);
+
+      if (leftVisible)
+        expect(left.opacity.value, equals(1.0));
+      if (rightVisible)
+        expect(right.opacity.value, equals(1.0));
+
+      leftVisibleBefore = leftVisible;
+      rightVisibleBefore = rightVisible;
+
+      // Check that the handles' positions are correct (clamped within the
+      // viewport but not stuck).
+
+      final List<Positioned> positioned =
+        find.byType(Positioned).evaluate().map((Element e) => e.widget).cast<Positioned>().toList();
+
+      final Size viewport = renderEditable.size;
+
+      void testPosition(double pos, Symbol expected) {
+        if (expected == #left)
+          expect(pos, equals(0.0));
+        if (expected == #right)
+          expect(pos, equals(viewport.width));
+        if (expected == #middle)
+          expect(pos, inExclusiveRange(0.0, viewport.width));
+      }
+
+      testPosition(positioned[0].left, leftPosition);
+      testPosition(positioned[1].left, rightPosition);
+    }
+
+    // Select the first word. Both handles should be visible.
+    await tester.tapAt(const Offset(20, 10));
+    renderEditable.selectWord(cause: SelectionChangedCause.longPress);
+    await tester.pump();
+    await verifyVisibility(true, #left, true, #middle);
+
+    // Drag the text slightly so the first word is partially visible. Only the
+    // right handle should be visible.
+    scrollable.controller.jumpTo(20.0);
+    await verifyVisibility(false, #left, true, #middle);
+
+    // Drag the text all the way to the left so the first word is not visible at
+    // all (and the second word is fully visible). Both handles should be
+    // invisible now.
+    scrollable.controller.jumpTo(200.0);
+    await verifyVisibility(false, #left, false, #left);
+
+    // Tap to unselect.
+    await tester.tap(find.byKey(editableTextKey));
+    await tester.pump();
+
+    // Now that the second word has been dragged fully into view, select it.
+    await tester.tapAt(const Offset(80, 10));
+    renderEditable.selectWord(cause: SelectionChangedCause.longPress);
+    await tester.pump();
+    await verifyVisibility(true, #middle, true, #middle);
+
+    // Drag the text slightly to the right. Only the left handle should be
+    // visible.
+    scrollable.controller.jumpTo(150);
+    await verifyVisibility(true, #middle, false, #right);
+
+    // Drag the text all the way to the right, so the second word is not visible
+    // at all. Again, both handles should be invisible.
+    scrollable.controller.jumpTo(0);
+    await verifyVisibility(false, #right, false, #right);
+  });
 }
 
 class MockTextSelectionControls extends Mock implements TextSelectionControls {}