MS в своей документации пишет: "Only a top-level window can be an active window. When the user is working with a child window, the system activates the top-level parent window associated with the child window".

И в той же документации:

WM_CHILDACTIVATE message: "Sent to a child window when the user clicks the window's title bar or when the window is activated, moved, or sized".

WM_MOUSEACTIVATE message: "The parent window determines whether to activate the child window. If it activates the child window, the parent window should return..."

Что здесь понимается под "активацией" дочернего окна?