Fix toggling of :active pseudoclass on elements

Only toggle :active pseudoclass on left mouse button clicks, ignore other mouse button clicks

Source/Core/Element.cpp

@@ -1564,7 +1564,8 @@ void Element::OnReferenceDeactivate()
 
 void Element::ProcessEvent(Event& event)
 {
-	if (event == MOUSEDOWN && IsPointWithinElement(Vector2f(event.GetParameter< float >("mouse_x", 0), event.GetParameter< float >("mouse_y", 0))))
+	if (event == MOUSEDOWN && IsPointWithinElement(Vector2f(event.GetParameter< float >("mouse_x", 0), event.GetParameter< float >("mouse_y", 0))) &&
+		event.GetParameter< int >("button", 0) == 0)
 		SetPseudoClass("active", true);
 
 	if (event == MOUSESCROLL)