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MSDN:
WM_WINDOWPOSCHANGING Notification
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The WM_WINDOWPOSCHANGING message is sent to a window whose size, position, or place in the Z order is about to change as a result of a call to the SetWindowPos function or another window-management function.
A window receives this message through its WindowProc function.
Syntax
WM_WINDOWPOSCHANGING
WPARAM wParam
LPARAM lParam;
Parameters
wParam
This parameter is not used.
lParam
Pointer to a WINDOWPOS structure that contains information about the window's new size and position.
Return Value
If an application processes this message, it should return zero.