Nonclient Area Mouse Messages
A window receives a nonclient area mouse message when a mouse event occurs in
any part of a window except the client area. A window's
nonclient area consists of its border, menu bar, title bar, scroll bar, System menu (also
called the Control menu), Minimize button, and Maximize button.
Windows generates nonclient area messages primarily for its own use. For
example, Windows uses nonclient area messages to change the cursor to a two-headed
arrow when the cursor hot spot moves into a window's border. A window must pass
nonclient area mouse messages to the
DefWindowProc function to take advantage of the built-in mouse interface found in Windows.
There is a corresponding nonclient area mouse message for each client area
mouse message. The names of these messages are similar except that the named
constants for the nonclient area messages include the letters "NC". For example,
moving the cursor in the nonclient area generates a
WM_NCMOUSEMOVE message, and pressing the left mouse button while the cursor is in the
nonclient area generates a
WM_NCLBUTTONDOWN message.
The
lParam parameter of a nonclient area mouse message is a
POINTS structure that contains the x- and y-coordinates of the cursor hot spot.
Unlike coordinates of client area mouse messages, the coordinates are given in
screen coordinates rather than client coordinates. In the screen coordinate system, all points
on the screen are relative to the coordinates (0,0) of the upper left corner of
the screen.
The
wParam parameter contains a
hit-test value, a value that indicates where in the nonclient area the mouse event occurred.
The following section explains the purpose of hit-test values.
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