fix(textbox): added SelectionBeforeKeyDown to the Skia TextBox hierarchy

This deals with idiosyncrasies of native behaviour on different Skia targets.

src/SamplesApp/UITests.Shared/Windows_UI_Xaml_Controls/TextBox/TextBox_Keyboard_Navigation.xaml

@@ -19,11 +19,11 @@
             <ColumnDefinition Width="350"/>
             <ColumnDefinition Width="350"/>
         </Grid.ColumnDefinitions>
-        <TextBox Grid.Column="0" Grid.Row="0" Text="Lorem"/>
+        <PasswordBox Grid.Column="0" Grid.Row="0" Password="Lorem"/>
         <TextBox Grid.Column="1" Grid.Row="0" Text="ipsum"/>
         <TextBox Grid.Column="2" Grid.Row="0" Text="dolor"/>
         <TextBox Grid.Column="0" Grid.Row="1" Text="sit"/>
-        <TextBox Grid.Column="1" Grid.Row="1" Text="amet"/>
+        <PasswordBox Grid.Column="1" Grid.Row="1" Password="amet"/>
         <TextBox Grid.Column="2" Grid.Row="1" Text="consectetur"/>
         <Button Grid.Column="0" Grid.Row="2" Width="80" Height="30" Content="Clear" Click="Button_OnClick"/>
         <Button Grid.Column="1" Grid.Row="2"  Width="80" Height="30" Content="Clear" Click="Button_OnClick"/>

src/Uno.UI.Runtime.Skia.Gtk/UI/Controls/GtkTextBoxView.cs

@@ -73,6 +73,9 @@ public void RemoveFromTextInputLayer()
 
 	public abstract (int start, int length) Selection { get; set; }
 
+	// On Gtk, KeyDown is fired before Selection is updated, so nothing special needs to be done.
+	public (int start, int length) SelectionBeforeKeyDown => Selection;
+
 	public abstract bool IsCompatible(TextBox textBox);
 
 	public abstract IDisposable ObserveTextChanges(EventHandler onChanged);

src/Uno.UI.Runtime.Skia.Wpf/Extensions/UI/Xaml/Controls/PasswordTextBoxView.cs

@@ -2,6 +2,7 @@
 
 using System;
 using System.Windows;
+using System.Windows.Input;
 using Uno.Disposables;
 using Uno.UI.Runtime.Skia.Wpf.UI.Controls;
 using WpfElement = System.Windows.UIElement;
@@ -18,6 +19,7 @@ internal class PasswordTextBoxView : WpfTextBoxView
 	private readonly WpfGrid _grid = new();
 	private readonly SerialDisposable _visibleControlTextChangedSubscription = new();
 	private EventHandler? _textChangedWatcher;
+	private (int start, int length) _selectionBeforeKeyDown;
 
 	public PasswordTextBoxView()
 	{
@@ -28,6 +30,9 @@ public PasswordTextBoxView()
 		_passwordBox.Visibility = Visibility.Collapsed;
 		_grid.Children.Add(_passwordBox);
 		_grid.Children.Add(_textBox);
+
+		_textBox.PreviewKeyDown += OnPreviewKeyDown;
+		_passwordBox.PreviewKeyDown += OnPreviewKeyDown;
 	}
 
 	public override string Text
@@ -47,6 +52,12 @@ public override (int start, int length) Selection
 		get => (_textBox.SelectionStart, _textBox.SelectionLength);
 		set => (_textBox.SelectionStart, _textBox.SelectionLength) = value;
 	}
+
+	public override (int start, int length) SelectionBeforeKeyDown
+	{
+		get => (_selectionBeforeKeyDown.start, _selectionBeforeKeyDown.length);
+		protected set => (_selectionBeforeKeyDown.start, _selectionBeforeKeyDown.length) = value;
+	}
 
 	public override bool IsCompatible(Windows.UI.Xaml.Controls.TextBox textBox) => textBox is Windows.UI.Xaml.Controls.PasswordBox;
 
@@ -118,4 +129,10 @@ private void ObserveVisibleControlTextChanges()
 	private void OnCommonTextChanged() => _textChangedWatcher?.Invoke(this, EventArgs.Empty);
 
 	private WpfElement GetDisplayedElement() => _textBox.Visibility == Visibility.Visible ? _textBox : _passwordBox;
+
+	private void OnPreviewKeyDown(object sender, KeyEventArgs e)
+	{
+		// On WPF, KeyDown is fired AFTER Selection is already changed to the new value
+		SelectionBeforeKeyDown = Selection;
+	}
 }

src/Uno.UI.Runtime.Skia.Wpf/Extensions/UI/Xaml/Controls/TextTextBoxView.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Windows;
 using System.Windows.Controls;
+using System.Windows.Input;
 using Uno.Disposables;
 using Uno.UI.Runtime.Skia.Wpf.UI.Controls;
 using Windows.UI.Xaml.Controls;
@@ -14,9 +15,11 @@ namespace Uno.UI.Runtime.Skia.Wpf.Extensions.UI.Xaml.Controls;
 internal class TextTextBoxView : WpfTextBoxView
 {
 	private readonly WpfTextViewTextBox _textBox = new();
+	private (int start, int length) _selectionBeforeKeyDown;
 
 	public TextTextBoxView()
 	{
+		_textBox.PreviewKeyDown += OnPreviewKeyDown;
 	}
 
 	public override string Text
@@ -33,6 +36,12 @@ public override (int start, int length) Selection
 		set => (_textBox.SelectionStart, _textBox.SelectionLength) = value;
 	}
 
+	public override (int start, int length) SelectionBeforeKeyDown
+	{
+		get => (_selectionBeforeKeyDown.start, _selectionBeforeKeyDown.length);
+		protected set => (_selectionBeforeKeyDown.start, _selectionBeforeKeyDown.length) = value;
+	}
+
 	public override void SetFocus() => _textBox.Focus();
 
 	public override bool IsCompatible(Windows.UI.Xaml.Controls.TextBox textBox) => textBox is not PasswordBox;
@@ -49,4 +58,10 @@ public override void UpdateProperties(Windows.UI.Xaml.Controls.TextBox winUIText
 		SetControlProperties(_textBox, winUITextBox);
 		SetTextBoxProperties(_textBox, winUITextBox);
 	}
+
+	private void OnPreviewKeyDown(object sender, KeyEventArgs e)
+	{
+		// On WPF, KeyDown is fired AFTER Selection is already changed to the new value
+		SelectionBeforeKeyDown = Selection;
+	}
 }

src/Uno.UI.Runtime.Skia.Wpf/Extensions/UI/Xaml/Controls/WpfTextBoxView.cs

@@ -25,6 +25,8 @@ protected WpfTextBoxView()
 	public bool IsDisplayed => RootElement.Parent is not null;
 
 	public abstract (int start, int length) Selection { get; set; }
+
+	public abstract (int start, int length) SelectionBeforeKeyDown { get; protected set; }
 
 	/// <summary>
 	/// Represents the root element of the input layout.

src/Uno.UI/UI/Xaml/Controls/TextBox/OverlayTextBoxView/IOverlayTextBoxView.skia.cs

@@ -14,6 +14,12 @@ internal interface IOverlayTextBoxView
 
 	(int start, int length) Selection { get; set; }
 
+	/// <summary>
+	/// On some platforms (namely Skia.WPF) KeyDown is fired after Selection is already set to the new value.
+	/// This property is provided to allow access to the selection value right before KeyDown.
+	/// </summary>
+	(int start, int length) SelectionBeforeKeyDown { get; }
+
 	/// <summary>
 	/// Returns a value indicating whether this TextBoxView is compatible with the given TextBox state.
 	/// </summary>

src/Uno.UI/UI/Xaml/Controls/TextBox/OverlayTextBoxView/IOverlayTextBoxViewExtension.skia.cs

@@ -28,6 +28,10 @@ internal interface IOverlayTextBoxViewExtension
 	int GetSelectionStart();
 
 	int GetSelectionLength();
+
+	int GetSelectionStartBeforeKeyDown();
+
+	int GetSelectionLengthBeforeKeyDown();
 
 	void UpdateProperties();
 }

src/Uno.UI/UI/Xaml/Controls/TextBox/OverlayTextBoxView/OverlayTextBoxViewExtension.skia.cs

@@ -216,6 +216,11 @@ public int GetSelectionLength()
 			_selectionLengthCache ?? 0 :
 			_textBoxView?.Selection.length ?? 0;
 	}
+
+	public int GetSelectionStartBeforeKeyDown() => _textBoxView!.SelectionBeforeKeyDown.start;
+
+	public int GetSelectionLengthBeforeKeyDown() => _textBoxView!.SelectionBeforeKeyDown.length;
+
 	private void EnsureTextBoxView(TextBox textBox)
 	{
 		if (_textBoxView is null ||

src/Uno.UI/UI/Xaml/Controls/TextBox/TextBox.cs

@@ -971,7 +971,7 @@ protected override void OnKeyDown(KeyRoutedEventArgs args)
 					}
 					break;
 				case VirtualKey.Down:
-					if (SelectionStart != Text.Length)
+					if (TextBoxView.SelectionBeforeKeyDown.start != Text.Length)
 					{
 						SelectionStart = Text.Length;
 						args.Handled = true;
@@ -982,13 +982,13 @@ protected override void OnKeyDown(KeyRoutedEventArgs args)
 					}
 					break;
 				case VirtualKey.Left:
-					if (SelectionStart != 0)
+					if (TextBoxView.SelectionBeforeKeyDown.start != 0)
 					{
 						args.Handled = true;
 					}
 					break;
 				case VirtualKey.Right:
-					if (SelectionStart != Text.Length)
+					if (TextBoxView.SelectionBeforeKeyDown.start != Text.Length)
 					{
 						args.Handled = true;
 					}

src/Uno.UI/UI/Xaml/Controls/TextBox/TextBoxView.skia.cs

@@ -33,6 +33,9 @@ public TextBoxView(TextBox textBox)
 			}
 		}
 
+		public (int start, int length) SelectionBeforeKeyDown =>
+			(_textBoxExtension!.GetSelectionStartBeforeKeyDown(), _textBoxExtension.GetSelectionLengthBeforeKeyDown());
+
 		internal IOverlayTextBoxViewExtension? Extension => _textBoxExtension;
 
 		public TextBox? TextBox