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Nonclient Area
Windows sends a WM_NCPAINT message to the window whenever a part of the nonclient area of the window,
such as the title bar, menu bar, or window frame, must be updated. Windows may
also send other messages to direct a window to update a portion of its client
area; for example, when a window becomes active or inactive, it sends the WM_NCACTIVATE message to update its title bar. In general, processing these messages for
standard windows is not recommended, because the application must be able to draw
all the required parts of the nonclient area for the window. For this reason,
most applications pass these messages to DefWindowProc for default processing.
An application that creates custom nonclient areas for its windows must
process these messages. When doing so, the application must use a window DC to carry
out drawing in the window. The window DC enables the application to draw in all portions of the window, including the
nonclient area. An application retrieves a window DC by using the GetWindowDC or GetDCEx function and, when drawing is complete, must release the window DC by using
the ReleaseDC function.
Windows maintains an update region for the nonclient area. When an application
receives a WM_NCPAINT message, the wParam parameter contains the handle of a region defining the dimensions of the
update region. The application can use the handle to combine the update region with
the clipping region for the window DC. Windows does not automatically combine
the update region when retrieving the window DC unless the application uses GetDCEx and specifies both the region handle and the DCX_INTERSECTRGN flag. If the
application does not combine the update region, only drawing operations that
would otherwise extend outside the window are clipped. The application is not
responsible for clearing the update region, regardless of whether it uses the
region.
If an application processes the WM_NCACTIVATE message, after processing it
must return TRUE to direct Windows to complete the change of active window. If the
window is minimized when the application receives the WM_NCACTIVATE message,
it should pass the message to DefWindowProc. In such cases, the default function redraws the label for the icon.
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