Added UI Unit Test to guard against regression.

Author: puppetsw

UITests/UITests.Tests.Shared/Controls/TextBoxMaskTest.cs

@@ -52,5 +52,31 @@ public void TestTextBoxMaskBinding_Property()
 
             Verify.AreEqual(newValue, textBox.GetText());
         }
+
+        [TestMethod]
+        [TestPage("TextBoxMaskTestPage")]
+        public void TestTextBoxMaskBinding_TextChanging_RegressionCheck()
+        {
+            var initialValue = FindElement.ById<TextBlock>("InitialValueTextBlock").GetText();
+            var textBox = FindElement.ById<Edit>("TextBox");
+
+            Verify.AreEqual(initialValue, textBox.GetText());
+
+            var setEmptyButton = FindElement.ById<Button>("SetEmptyButton");
+
+            setEmptyButton.Click();
+            Wait.ForIdle();
+
+            // Previously, if the bound value of TextBox with a mask was set to an empty
+            // string. It would display the old values after text was entered by the user.
+            // PR #4048 should fix this issue.
+            textBox.SendKeys("0");
+
+            // The user entering "0" key at the start position of the TextBox would result
+            // in the following value prior to PR #4048.
+            const string incorrectValue = "02:50:59";
+
+            Verify.AreNotEqual(incorrectValue, textBox.GetText());
+        }
     }
 }

UITests/UITests.Tests.Shared/Controls/TextBoxMaskTestPage.xaml

@@ -12,6 +12,8 @@
             <RowDefinition Height="Auto" />
             <RowDefinition Height="10" />
             <RowDefinition Height="Auto" />
+            <RowDefinition Height="10" />
+            <RowDefinition Height="Auto" />
         </Grid.RowDefinitions>
         <Grid.ColumnDefinitions>
             <ColumnDefinition Width="Auto" />
@@ -43,5 +45,10 @@
                    Grid.Column="2"
                    HorizontalAlignment="Center"
                    Text="{x:Bind NewValue}" />
+        <Button x:Name="SetEmptyButton"
+                Grid.Row="4"
+                Grid.Column="0"
+                Click="SetEmptyButton_Click"
+                Content="Set Target Value Empty" />
     </Grid>
 </Page>

UITests/UITests.Tests.Shared/Controls/TextBoxMaskTestPage.xaml.cs

@@ -38,5 +38,11 @@ private void ChangeButton_Click(object sender, RoutedEventArgs e)
             Value = NEW_VALUE;
             Log.Comment("Value Changed to {0}", Value);
         }
+
+        private void SetEmptyButton_Click(object sender, RoutedEventArgs e)
+        {
+            Value = string.Empty;
+            Log.Comment("Value Changed to {0}", Value);
+        }
     }
 }