fix(TextBox): fix MaxLength to work correctly with paste

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_TextBox.skia.cs

@@ -4160,6 +4160,41 @@ public async Task When_Setting_Short_Text_And_Previous_Selection_Is_OutOfBounds(
 			SUT.RaiseEvent(UIElement.KeyUpEvent, new KeyRoutedEventArgs(SUT, VirtualKey.Escape, VirtualKeyModifiers.None));
 		}
 
+		[TestMethod]
+		public async Task When_MaxLine_Paste()
+		{
+			using var _ = new TextBoxFeatureConfigDisposable();
+
+			var SUT = new TextBox
+			{
+				MaxLength = 10,
+				Text = "0123456789",
+				SelectionStart = 4,
+				SelectionLength = 2
+			};
+
+			WindowHelper.WindowContent = SUT;
+
+			await WindowHelper.WaitForIdle();
+			await WindowHelper.WaitForLoaded(SUT);
+
+			SUT.Focus(FocusState.Programmatic);
+			await WindowHelper.WaitForIdle();
+
+			var dp = new DataPackage();
+			var text = "abcdefgh";
+			dp.SetText(text);
+			Clipboard.SetContent(dp);
+			await WindowHelper.WaitForIdle();
+
+			SUT.PasteFromClipboard();
+			await WindowHelper.WaitForIdle();
+
+			Assert.AreEqual("0123ab6789", SUT.Text);
+			Assert.AreEqual(6, SUT.SelectionStart);
+			Assert.AreEqual(0, SUT.SelectionLength);
+		}
+
 		private static bool HasColorInRectangle(RawBitmap screenshot, Rectangle rect, Color expectedColor)
 		{
 			for (var x = rect.Left; x < rect.Right; x++)

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

@@ -417,20 +417,18 @@ private object CoerceText(object baseValue)
 
 			if (MaxLength > 0 && baseString.Length > MaxLength)
 			{
+				// Reject the new string if it's longer than the MaxLength
+#if __SKIA__
+				_pendingSelection = null;
+#endif
 				return DependencyProperty.UnsetValue;
 			}
 
-#if __SKIA__
 			if (!AcceptsReturn)
 			{
 				baseString = GetFirstLine(baseString);
-				if (_pendingSelection is { } selection)
-				{
-					var start = Math.Min(selection.start, baseString.Length);
-					var end = Math.Min(selection.start + selection.length, baseString.Length);
-					_pendingSelection = (start, end - start);
-				}
 			}
+#if __SKIA__
 			else if (_isSkiaTextBox)
 			{
 				// WinUI replaces all \n's and and \r\n's by \r. This is annoying because
@@ -439,10 +437,13 @@ private object CoerceText(object baseValue)
 				// the native input and the managed representation.
 				baseString = RemoveLF(baseString);
 			}
-#else
-			if (!AcceptsReturn)
+
+			// make sure this coercion doesn't cause the pending selection to be out of range
+			if (_pendingSelection is { } selection2)
 			{
-				baseString = GetFirstLine(baseString);
+				var start = Math.Min(selection2.start, baseString.Length);
+				var end = Math.Min(selection2.start + selection2.length, baseString.Length);
+				_pendingSelection = (start, end - start);
 			}
 #endif
 
@@ -1404,15 +1405,21 @@ internal void PasteFromClipboard(string clipboardText)
 			var selectionStart = SelectionStart;
 			var selectionLength = SelectionLength;
 			var currentText = Text;
+			var adjustedClipboardText = clipboardText;
 
 			if (selectionLength > 0)
 			{
 				currentText = currentText.Remove(selectionStart, selectionLength);
 			}
 
-			currentText = currentText.Insert(selectionStart, clipboardText);
+			if (MaxLength > 0)
+			{
+				var clipboardRangeToBePasted = Math.Max(0, Math.Min(clipboardText.Length, MaxLength - currentText.Length));
+				adjustedClipboardText = clipboardText[..clipboardRangeToBePasted];
+			}
 
-			PasteFromClipboardPartial(clipboardText, selectionStart, selectionLength, currentText);
+			currentText = currentText.Insert(selectionStart, adjustedClipboardText);
+			PasteFromClipboardPartial(adjustedClipboardText, selectionStart, currentText);
 
 #if __SKIA__
 			try
@@ -1442,7 +1449,7 @@ internal void PasteFromClipboard(string clipboardText)
 #endif
 		}
 
-		partial void PasteFromClipboardPartial(string clipboardText, int selectionStart, int selectionLength, string newText);
+		partial void PasteFromClipboardPartial(string adjustedClipboardText, int selectionStart, string newText);
 
 		/// <summary>
 		/// Copies the selected content to the OS clipboard.

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

@@ -1212,7 +1212,7 @@ private string RemoveLF(string baseString)
 		return baseString;
 	}
 
-	partial void PasteFromClipboardPartial(string clipboardText, int selectionStart, int selectionLength, string newText)
+	partial void PasteFromClipboardPartial(string adjustedClipboardText, int selectionStart, string newText)
 	{
 		if (_isSkiaTextBox)
 		{
@@ -1227,7 +1227,7 @@ partial void PasteFromClipboardPartial(string clipboardText, int selectionStart,
 				CommitAction(new ReplaceAction(Text, newText, selectionStart));
 			}
 
-			_pendingSelection = (selectionStart + clipboardText.Length, 0);
+			_pendingSelection = (selectionStart + adjustedClipboardText.Length, 0);
 		}
 	}