fix: adjust arrow navigation in TextBox

Author: barjonp

src/Runtime/Runtime/System.Windows.Controls/TextBox.Navigation.cs

@@ -98,12 +98,46 @@ private bool NavigateInDirection(KeyEventArgs e)
             switch (e.Key)
             {
                 case Key.Left:
-                case Key.Up:
                     return CaretPosition > 0;
 
+                case Key.Up:
+                    {
+                        int caretPosition = CaretPosition;
+                        if (caretPosition == 0)
+                        {
+                            return false;
+                        }
+
+                        string text = _textViewHost.View.GetText();
+                        if (text.IndexOf('\n', 0, caretPosition) == -1 && text.IndexOf('\r', 0, caretPosition) == -1)
+                        {
+                            e.PreventDefault();
+                            e.Cancellable = false; // make sure that PreventDefault will not be called again
+                            return false;
+                        }
+                        return true;
+                    }
+
                 case Key.Right:
-                case Key.Down:
                     return CaretPosition < _textViewHost.View.GetText().Length;
+                
+                case Key.Down:
+                    {
+                        int caretPosition = CaretPosition;
+                        string text = _textViewHost.View.GetText();
+                        if (caretPosition == text.Length)
+                        {
+                            return false;
+                        }
+
+                        if (text.IndexOf('\n', caretPosition) == -1 && text.IndexOf('\r', caretPosition) == -1)
+                        {
+                            e.PreventDefault();
+                            e.Cancellable = false; // make sure that PreventDefault will not be called again
+                            return false;
+                        }
+                        return true;
+                    }
 
                 default:
                     Debug.Assert(false);

src/Runtime/Runtime/System.Windows.input/InputManager.cs

@@ -373,10 +373,9 @@ private void ProcessOnKeyDown(UIElement uie, object jsEventArg)
 
         keyboardTarget.RaiseEvent(e);
 
-        if (e.PreventDefault)
+        if (e.Handled && e.Cancellable)
         {
-            OpenSilver.Interop.ExecuteJavaScriptVoid(
-                $"{CSHTML5.INTERNAL_InteropImplementation.GetVariableStringForJS(jsEventArg)}.preventDefault()");
+            e.PreventDefault();
         }
     }
 

src/Runtime/Runtime/System.Windows.input/KeyRoutedEventArgs.cs

@@ -61,7 +61,14 @@ public bool Handled
 
         internal bool Cancellable { get; set; } = true;
 
-        internal bool PreventDefault => Handled && Cancellable;
+        internal void PreventDefault()
+        {
+            if (UIEventArg != null)
+            {
+                OpenSilver.Interop.ExecuteJavaScriptVoid(
+                    $"{INTERNAL_InteropImplementation.GetVariableStringForJS(UIEventArg)}.preventDefault();");
+            }
+        }
 
         // Returns:
         //     A system value that indicates the code for the key referenced by the event.