fix(SelectableTextBlock): fix SelectableTextBlock selection for centered and right-aligned text (#21369)

* fix(SelectableTextBlock): fix SelectableTextBlock selection for centered and right-aligned text

* Fix failing unit tests

* Let TextLayout handle out-of-bounds hit testing

---------

Co-authored-by: Julien Lebosquain <julien@lebosquain.net>

samples/ControlCatalog/Pages/TextBlockPage.xaml

@@ -119,15 +119,20 @@
         </StackPanel>
       </Border>
       <Border>
-        <SelectableTextBlock Margin="10" TextWrapping="Wrap">
-          This <Span FontWeight="Bold">is</Span> a
-          <Span Background="Silver" Foreground="Maroon">TextBlock</Span>
-          with <Span TextDecorations="Underline">several</Span>
-          <Span FontStyle="Italic">Span</Span> elements,
-          <Span Foreground="Blue">
-            using a <Bold>variety</Bold> of <Italic>styles</Italic>
-          </Span>.
-        </SelectableTextBlock>
+        <StackPanel Spacing="8">
+          <SelectableTextBlock Margin="10" TextWrapping="Wrap">
+            This <Span FontWeight="Bold">is</Span> a
+            <Span Background="Silver" Foreground="Maroon">TextBlock</Span>
+            with <Span TextDecorations="Underline">several</Span>
+            <Span FontStyle="Italic">Span</Span> elements,
+            <Span Foreground="Blue">
+              using a <Bold>variety</Bold> of <Italic>styles</Italic>
+            </Span>.
+          </SelectableTextBlock>
+          <SelectableTextBlock Text="Left aligned text" TextAlignment="Left" />
+          <SelectableTextBlock Text="Center aligned text" TextAlignment="Center" />
+          <SelectableTextBlock Text="Right aligned text and wrap text" TextAlignment="Right" TextWrapping="Wrap" />
+        </StackPanel>
       </Border>
       <Border>
         <TextBlock FontFamily="Times New Roman">

src/Avalonia.Controls/SelectableTextBlock.cs

@@ -451,10 +451,6 @@ namespace Avalonia.Controls
 
                 var point = e.GetPosition(this) - new Point(padding.Left, padding.Top);
 
-                point = new Point(
-                    MathUtilities.Clamp(point.X, 0, Math.Max(TextLayout.WidthIncludingTrailingWhitespace, 0)),
-                    MathUtilities.Clamp(point.Y, 0, Math.Max(TextLayout.Height, 0)));
-
                 var hit = TextLayout.HitTestPoint(point);
                 var textPosition = hit.TextPosition;
 

tests/Avalonia.Controls.UnitTests/SelectableTextBlockTests.cs

@@ -1,4 +1,5 @@
-using System.Linq;
+using System;
+using System.Linq;
 using Avalonia.Controls.Documents;
 using Avalonia.Media;
 using Avalonia.Media.TextFormatting;
@@ -49,6 +50,46 @@ namespace Avalonia.Controls.UnitTests
             }
         }
 
+        [Theory]
+        [InlineData(TextAlignment.Center)]
+        [InlineData(TextAlignment.Right)]
+        public void Dragging_Selection_Should_Reach_End_Of_Text_When_Text_Is_Aligned(TextAlignment textAlignment)
+        {
+            using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface))
+            {
+                var target = new SelectableTextBlock
+                {
+                    Width = 200,
+                    Text = "Aligned text",
+                    TextAlignment = textAlignment
+                };
+
+                var root = new TestRoot(target)
+                {
+                    ClientSize = new Size(300, 100)
+                };
+
+                root.Measure(root.ClientSize);
+                root.Arrange(new Rect(root.ClientSize));
+                root.ExecuteInitialLayoutPass();
+
+                var firstCharacterBounds = target.TextLayout.HitTestTextPosition(0);
+                var lastCharacterBounds = target.TextLayout.HitTestTextPosition(target.Text!.Length - 1);
+                var mouse = new MouseTestHelper();
+                var startPoint = new Point(
+                    firstCharacterBounds.X + firstCharacterBounds.Width / 2,
+                    firstCharacterBounds.Y + firstCharacterBounds.Height / 2);
+                var endPoint = new Point(
+                    Math.Min(target.Bounds.Width - 1, lastCharacterBounds.Right + 10),
+                    lastCharacterBounds.Y + lastCharacterBounds.Height / 2);
+
+                mouse.Down(target, position: target.TranslatePoint(startPoint, root));
+                mouse.Move(target, position: target.TranslatePoint(endPoint, root).GetValueOrDefault());
+
+                Assert.Equal(target.Text!.Length, Math.Max(target.SelectionStart, target.SelectionEnd));
+            }
+        }
+
         [Fact]
         public void SelectionForeground_Should_Not_Reset_Run_Typeface_And_Style()
         {