Merge pull request #1669 from AvaloniaUI/feature/tooltips-obey-screen-edges

ensure tooltips fit inside screen edges.

src/Avalonia.Controls/ToolTip.cs

@@ -234,11 +234,12 @@ namespace Avalonia.Controls
         {
             Close();
 
-            _popup = new PopupRoot { Content = this };
+            _popup = new PopupRoot { Content = this,  };
             ((ISetLogicalParent)_popup).SetParent(control);
             _popup.Position = Popup.GetPosition(control, GetPlacement(control), _popup,
                 GetHorizontalOffset(control), GetVerticalOffset(control));
             _popup.Show();
+            _popup.SnapInsideScreenEdges();
         }
 
         private void Close()