WM_NCMBUTTONDBLCLK top-level constant
int const WM_NCMBUTTONDBLCLK
Posted when the user double-clicks the middle mouse button while the cursor is within the nonclient area of a window. This message is posted to the window that contains the cursor. If a window has captured the mouse, this message is not posted.
const WM_NCMBUTTONDBLCLK = 0x00A9;