fix TextBox multiline selection with up/down keys till start/end of text (#18746)

* fix: TextBox multiline selection with up/down keys till start/end of text

* refactor to preserve public API of TextPresenter

* refactor and cleanup

src/Avalonia.Controls/TextBox.cs

@@ -1464,59 +1464,23 @@ namespace Avalonia.Controls
                         break;
 
                     case Key.Up:
+                        selection = DetectSelection();
+                        MoveVertical(LogicalDirection.Backward, selection);
+                        if (caretIndex != _presenter.CaretIndex)
                         {
-                            selection = DetectSelection();
-
-                            if (!selection && SelectionStart != SelectionEnd)
-                            {
-                                ClearSelectionAndMoveCaretToTextPosition(LogicalDirection.Backward);
-                            }
-
-                            _presenter.MoveCaretVertical(LogicalDirection.Backward);
-
-                            if (caretIndex != _presenter.CaretIndex)
-                            {
-                                movement = true;
-                            }
-
-                            if (selection)
-                            {
-                                SetCurrentValue(SelectionEndProperty, _presenter.CaretIndex);
-                            }
-                            else
-                            {
-                                SetCurrentValue(CaretIndexProperty, _presenter.CaretIndex);
-                            }
-
-                            break;
+                            movement = true;
                         }
+                        break;
+
                     case Key.Down:
+                        selection = DetectSelection();
+                        MoveVertical(LogicalDirection.Forward, selection);
+                        if (caretIndex != _presenter.CaretIndex)
                         {
-                            selection = DetectSelection();
-
-                            if (!selection && SelectionStart != SelectionEnd)
-                            {
-                                ClearSelectionAndMoveCaretToTextPosition(LogicalDirection.Forward);
-                            }
-
-                            _presenter.MoveCaretVertical();
-
-                            if (caretIndex != _presenter.CaretIndex)
-                            {
-                                movement = true;
-                            }
-
-                            if (selection)
-                            {
-                                SetCurrentValue(SelectionEndProperty, _presenter.CaretIndex);
-                            }
-                            else
-                            {
-                                SetCurrentValue(CaretIndexProperty, _presenter.CaretIndex);
-                            }
-
-                            break;
+                            movement = true;
                         }
+                        break;
+
                     case Key.Back:
                         {
                             SnapshotUndoRedo();
@@ -2038,6 +2002,50 @@ namespace Avalonia.Controls
             }
         }
 
+        private void MoveVertical(LogicalDirection direction, bool isSelecting)
+        {
+            if (_presenter is null)
+            {
+                return;
+            }
+
+            if (isSelecting)
+            {
+                var oldCaretIndex = _presenter.CaretIndex;
+                _presenter.MoveCaretVertical(direction);
+                var newCaretIndex = _presenter.CaretIndex;
+
+                if (oldCaretIndex == newCaretIndex)
+                {
+                    var text = Text ?? string.Empty;
+
+                    // caret did not move while we are selecting so we could not move to previous/next line,
+                    // but check if we are already at the 'boundary' of the text
+                    if (direction == LogicalDirection.Forward && newCaretIndex < text.Length)
+                    {
+                        _presenter.MoveCaretToTextPosition(text.Length);
+                    }
+                    else if (direction == LogicalDirection.Backward && newCaretIndex > 0)
+                    {
+                        _presenter.MoveCaretToTextPosition(0);
+                    }
+                }
+
+                SetCurrentValue(SelectionEndProperty, _presenter.CaretIndex);
+            }
+            else
+            {
+                if (SelectionStart != SelectionEnd)
+                {
+                    ClearSelectionAndMoveCaretToTextPosition(direction);
+                }
+
+                _presenter.MoveCaretVertical(direction);
+
+                SetCurrentValue(CaretIndexProperty, _presenter.CaretIndex);
+            }
+        }
+
         private void MoveHome(bool document)
         {
             if (_presenter is null)

tests/Avalonia.Controls.UnitTests/TextBoxTests.cs

@@ -1875,6 +1875,68 @@ namespace Avalonia.Controls.UnitTests
             }
         }
 
+        [Theory]
+        [InlineData(0)]
+        [InlineData(4)]
+        [InlineData(8)]
+        public void When_Selecting_Multiline_Selection_Should_Be_Extended_With_Up_Arrow_Key_Till_Start_Of_Text(int caretOffsetFromEnd)
+        {
+            using (UnitTestApplication.Start(Services))
+            {
+                var tb = new TextBox
+                {
+                    Template = CreateTemplate(),
+                    Text = """
+                           AAAAAA
+                           BBBB
+                           CCCCCCCC
+                           """,
+                    AcceptsReturn = true
+                };
+                tb.ApplyTemplate();
+                tb.Measure(Size.Infinity);
+                tb.CaretIndex = tb.Text.Length - caretOffsetFromEnd;
+
+                RaiseKeyEvent(tb, Key.Up, KeyModifiers.Shift);
+                RaiseKeyEvent(tb, Key.Up, KeyModifiers.Shift);
+                RaiseKeyEvent(tb, Key.Up, KeyModifiers.Shift);
+                RaiseKeyEvent(tb, Key.Up, KeyModifiers.Shift);
+
+                Assert.Equal(0, tb.SelectionEnd);
+            }
+        }
+
+        [Theory]
+        [InlineData(0)]
+        [InlineData(3)]
+        [InlineData(6)]
+        public void When_Selecting_Multiline_Selection_Should_Be_Extended_With_Down_Arrow_Key_Till_End_Of_Text(int caretOffsetFromStart)
+        {
+            using (UnitTestApplication.Start(Services))
+            {
+                var tb = new TextBox
+                {
+                    Template = CreateTemplate(),
+                    Text = """
+                           AAAAAA
+                           BBBB
+                           CCCCCCCC
+                           """,
+                    AcceptsReturn = true
+                };
+                tb.ApplyTemplate();
+                tb.Measure(Size.Infinity);
+                tb.CaretIndex = caretOffsetFromStart;
+
+                RaiseKeyEvent(tb, Key.Down, KeyModifiers.Shift);
+                RaiseKeyEvent(tb, Key.Down, KeyModifiers.Shift);
+                RaiseKeyEvent(tb, Key.Down, KeyModifiers.Shift);
+                RaiseKeyEvent(tb, Key.Down, KeyModifiers.Shift);
+
+                Assert.Equal(tb.Text.Length, tb.SelectionEnd);
+            }
+        }
+
         [Fact]
         public void TextBox_In_AdornerLayer_Will_Not_Cause_Collection_Modified_In_VisualLayerManager_Measure()
         {