When the system moves a window or changes its size, it can invalidate all or part of that window. The system attempts to preserve the contents of the window and copy them to the new position; however, if the window's size is increased, the window must fill the area exposed by the size change. If a window is moved from behind an overlapping window, any area formerly obscured by the other window must be drawn. In these cases, the system invalidates the exposed areas and sends a WM_PAINT message to the window.

An application can require that the system invalidate an entire window every time the window moves or changes size. To do this, the application sets the CS_SIZEREDRAW class style in the corresponding window class. Typically, this class style is selected for use in an application that uses a window's current size and position to determine how to draw the window. For example, a clock application always would draw the face of the clock so that it filled the window exactly.

An application also can explicitly specify which parts of the window to preserve during a move or size change. Before any change is made, the system sends a WM_CALCVALIDRECTS message to windows that do not have the style CS_SIZEREDRAW. This enables the window procedure to specify what part of the window to save and where to align it after the move or size change.