Topic: Whether event resize () is always caused at initiating discovery of a window?

In manual PB 10.5 it is written "Occurs when the user or a script opens or resizes the client area of a window" - i.e. it is unambiguous at discovery or change of the sizes of client area of a window the output agent of this event should be caused.
In practice in one case event resize () happens - and even earlier open (), and in other - does not happen at all, though the window appears (event open () happened). It turns out  with the description?
How precisely to define - in what cases event Resize () happens at the first discovery of a window and in what - is not present?
(Just in case: the First window was the earliest in the program, and the second - was caused already after, from the first).