The system sends WM_MOUSEMOVE messages to the window that is under the mouse pointer, or to the window that currently has captured the mouse, whenever the mouse pointer moves. This is useful for tracking the mouse pointer and changing its shape, based on its location in a window. For example, the mouse pointer changes shape when it passes over the size border of a standard frame window.

All standard control windows use WM_MOUSEMOVE messages to set the mouse-pointer shape. If an application handles WM_MOUSEMOVE messages in some situations but not others, unused messages should be passed to the WinDefWindowProc function to change the mouse-pointer shape.