As learnt in previous commits, the GUIManager methods that only operate on the topmost page are typically conceptually flawed as the topmost page might not be the intended page to work on by the event handling code.
This was the case for rP22200/D1701, rP22663/D2108, rP22922/D2302 for example.
Another example would be changing the NetClient GUIMessage code to push the GUI messages to all GUI pages having GUI objects subscribing to that event.
Another example use case would be the session GUI page and the summary dialog both subscribing to SimulationUpdate messages.