dont allow text selection to be lost when context menu is opened... i.e. for copy and paste to work.

src/Avalonia.Controls/ContextMenu.cs

@@ -280,9 +280,8 @@ namespace Avalonia.Controls
             }
 
             _popup.Child = this;
-            _popup.IsOpen = true;
-
             IsOpen = true;
+            _popup.IsOpen = true;
 
             RaiseEvent(new RoutedEventArgs
             {

src/Avalonia.Controls/TextBox.cs

@@ -410,10 +410,15 @@ namespace Avalonia.Controls
         protected override void OnLostFocus(RoutedEventArgs e)
         {
             base.OnLostFocus(e);
-            SelectionStart = 0;
-            SelectionEnd = 0;
+
+            if (ContextMenu == null || !ContextMenu.IsOpen)
+            {
+                SelectionStart = 0;
+                SelectionEnd = 0;
+                RevealPassword = false;
+            }
+            
             _presenter?.HideCaret();
-            RevealPassword = false;
         }
 
         protected override void OnTextInput(TextInputEventArgs e)