[This is preliminary documentation and subject to change.]
This event is generated in response to the topmost level object that has changed, not for any children it might contain. For example, if the user resizes a top-level window, USER generates this notification for the window, but not for the menu bar, title bar, scroll bars, or other objects that are also changing.
USER generates this event for windows that are changing. It does not do this for every nonfloating child window when the parent moves. However, if an application explicitly resizes child windows as a result of being sized, USER will generate multiple events for the resized children. Additionally, USER generates this event for two nonwindow system objects: the system caret and the cursor.
For this event notification, the idChild parameter of the WinEventProc callback function identifies the child object that changed.