WM_NCXBUTTONDOWN top-level constant
int
const WM_NCXBUTTONDOWN
Posted when the user presses the first or second X button while the cursor is in 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.
Implementation
const WM_NCXBUTTONDOWN = 0x00AB;