fix(textbox): BeforeTextChanging crashing ing in specific cases

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

@@ -427,6 +427,21 @@ private object CoerceText(object baseValue)
 			BeforeTextChanging?.Invoke(this, args);
 			if (args.Cancel)
 			{
+#if __SKIA__
+				if (_isSkiaTextBox)
+				{
+					// On WinUI, when a selection is canceled, the TextBox invokes a bunch of weird
+					// SelectionChanging events followed by a bunch of matching SelectionChanged.
+					// Probing for the value of SelectionStart and SelectionLength during these SelectionChanging
+					// events will give incorrect transient values and the SelectionChanged events will end up
+					// with the selection where it started (before the text change). Also, the direction of
+					// of the selection will be reset, i.e. if the selection end was "at the start", then it won't be
+					// so anymore.
+					// In Uno, we choose a simpler sequence. We just reset the selection direction (like WinUI) and
+					// we don't invoke any selection change events (since selection was in fact not changed).
+					_pendingSelection = (SelectionStart, SelectionLength);
+				}
+#endif
 				return DependencyProperty.UnsetValue;
 			}
 

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

@@ -140,7 +140,6 @@ private void TrySetCurrentlyTyping(bool newValue)
 		}
 		else
 		{
-			global::System.Diagnostics.CI.Assert(!_isSkiaTextBox || _selection.length == 0);
 			_historyIndex++;
 			_history.RemoveAllAt(_historyIndex);
 			_history.Add(new HistoryRecord(