Fixing popup placement

Gtk/Perspex.Gtk/PopupImpl.cs

@@ -14,13 +14,9 @@ namespace Perspex.Gtk
     {
         public PopupImpl()
             : base(WindowType.Popup)
-        {
-            this.DefaultSize = new Gdk.Size(640, 480);
-            this.Events = Gdk.EventMask.PointerMotionMask |
-                          Gdk.EventMask.ButtonPressMask |
-                          Gdk.EventMask.ButtonReleaseMask;
+        { 
         }
-
+        
         public void SetPosition(Point p)
         {
             this.Move((int)p.X, (int)p.Y);

Gtk/Perspex.Gtk/WindowImpl.cs

@@ -35,13 +35,12 @@ namespace Perspex.Gtk
         public WindowImpl(Gtk.WindowType type)
             : base(type)
         {
-            this.DefaultSize = new Gdk.Size(640, 480);
             this.Events = Gdk.EventMask.PointerMotionMask |
                           Gdk.EventMask.ButtonPressMask |
                           Gdk.EventMask.ButtonReleaseMask;
             this.windowHandle = new PlatformHandle(this.Handle, "GtkWindow");
         }
-
+        
         public Size ClientSize
         {
             get;
@@ -79,8 +78,10 @@ namespace Perspex.Gtk
 
         public Point PointToScreen(Point point)
         {
-            // TODO: Implement.
-            return new Point();
+            int x, y;
+            this.GdkWindow.GetDeskrelativeOrigin(out x, out y);
+            
+            return new Point(point.X + x, point.Y + y);
         }
 
         public void SetOwner(TopLevel owner)

Perspex.Controls/ToolTip.cs

@@ -116,8 +116,8 @@ namespace Perspex.Controls
                     ((ISetLogicalParent)popup).SetParent(control);
                 }
 
-                var cp = MouseDevice.Instance.GetPosition(control);
-                var position = control.PointToScreen(cp) + new Vector(0, 22);
+                var cp = MouseDevice.Instance?.GetPosition(control);
+                var position = control.PointToScreen(cp.HasValue ? cp.Value : new Point(0, 0)) + new Vector(0, 22);
 
                 ((ToolTip)popup.Content).Content = GetTip(control);
                 popup.SetPosition(position);