Implement ToolTip :open pseudo class

src/Avalonia.Controls/ToolTip.cs

@@ -204,13 +204,15 @@ namespace Avalonia.Controls
         private static void IsOpenChanged(AvaloniaPropertyChangedEventArgs e)
         {
             var control = (Control)e.Sender;
+            var newValue = (bool)e.NewValue;
+            ToolTip toolTip;
 
-            if ((bool)e.NewValue)
+            if (newValue)
             {
                 var tip = GetTip(control);
                 if (tip == null) return;
 
-                var toolTip = control.GetValue(ToolTipProperty);
+                toolTip = control.GetValue(ToolTipProperty);
                 if (toolTip == null || (tip != toolTip && tip != toolTip.Content))
                 {
                     toolTip?.Close();
@@ -223,9 +225,11 @@ namespace Avalonia.Controls
             }
             else
             {
-                var toolTip = control.GetValue(ToolTipProperty);
+                toolTip = control.GetValue(ToolTipProperty);
                 toolTip?.Close();
             }
+
+            toolTip?.UpdatePseudoClasses(newValue);
         }
 
         private void Open(Control control)
@@ -261,5 +265,10 @@ namespace Avalonia.Controls
                 pr.PlatformImpl.SetWindowManagerAddShadowHint(hint);
             }
         }
+
+        private void UpdatePseudoClasses(bool newValue)
+        {
+            PseudoClasses.Set(":open", newValue);
+        }
     }
 }