fix: Update text according to AcceptsReturn

src/Uno.UI.Tests/Windows_UI_XAML_Controls/TextBoxTests/Given_TextBox.cs

@@ -94,13 +94,13 @@ public void When_BeforeTextChanging_Cancel()
 			var textChangingCount = 0;
 			var beforeTextChangingCount = 0;
 			textBox.BeforeTextChanging += (tb, e) =>
-			  {
-				  beforeTextChangingCount++;
-				  if (e.NewText == "Papaya")
-				  {
-					  e.Cancel = true;
-				  }
-			  };
+			{
+				beforeTextChangingCount++;
+				if (e.NewText == "Papaya")
+				{
+					e.Cancel = true;
+				}
+			};
 			textBox.TextChanging += (tb, e) => textChangingCount++;
 			textBox.Text = "Chirimoya";
 			Assert.AreEqual("Chirimoya", textBox.Text);
@@ -116,6 +116,31 @@ public void When_BeforeTextChanging_Cancel()
 			Assert.AreEqual(2, beforeTextChangingCount);
 		}
 
+		[TestMethod]
+		public void When_Multi_Line_Text_And_Not_AcceptsReturn()
+		{
+			var textBox = new TextBox();
+			Assert.AreEqual(false, textBox.AcceptsReturn);
+			textBox.Text = "Hello\nWorld";
+			Assert.AreEqual("Hello", textBox.Text);
+
+			textBox.Text = "Hello\rWorld";
+			Assert.AreEqual("Hello", textBox.Text);
+		}
+
+		[TestMethod]
+		public void When_Multi_Line_Text_And_Not_AcceptsReturn_After_Text_Was_Set()
+		{
+			var textBox = new TextBox();
+
+			textBox.AcceptsReturn = true;
+			textBox.Text = "Hello\nWorld";
+			Assert.AreEqual("Hello\nWorld", textBox.Text);
+
+			textBox.AcceptsReturn = false;
+			Assert.AreEqual("Hello", textBox.Text);
+		}
+
 		public class MySource : System.ComponentModel.INotifyPropertyChanged
 		{
 			private string _sourceText;

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

@@ -168,6 +168,20 @@ public string Text
 			}
 		}
 
+		private static string GetSingleLine(string value)
+		{
+			for (int i = 0; i < value.Length; i++)
+			{
+				var c = value[i];
+				if (c == '\r' || c == '\n')
+				{
+					return value.Substring(0, i);
+				}
+			}
+
+			return value;
+		}
+
 		public static DependencyProperty TextProperty { get; } =
 			DependencyProperty.Register(
 				"Text",
@@ -265,14 +279,19 @@ private object CoerceText(object baseValue)
 				return DependencyProperty.UnsetValue;
 			}
 
+			if (!AcceptsReturn)
+			{
+				baseString = GetSingleLine(baseString);
+			}
+
 			var args = new TextBoxBeforeTextChangingEventArgs(baseString);
 			BeforeTextChanging?.Invoke(this, args);
 			if (args.Cancel)
 			{
 				return DependencyProperty.UnsetValue;
 			}
 
-			return baseValue;
+			return baseString;
 		}
 
 		#endregion
@@ -404,6 +423,16 @@ public bool AcceptsReturn
 
 		private void OnAcceptsReturnChanged(DependencyPropertyChangedEventArgs e)
 		{
+			if (e.NewValue is false)
+			{
+				var text = Text;
+				var singleLineText = GetSingleLine(text);
+				if (text != singleLineText)
+				{
+					Text = singleLineText;
+				}
+			}
+
 			OnAcceptsReturnChangedPartial(e);
 			UpdateButtonStates();
 		}