This message is not sent if this window has the CS_SIZEREDRAW style, indicating size-sensitive window content that must be totally redrawn if sized.
This enables the application to determine if the position of the window has changed as well as its size; this can aid alignment processing.
These rectangles can be modified by the window procedure to cause parts of the window to be redrawn and not preserved.
The window manager tries to preserve the screen image by copying the image described by the old rectangle into the image described by the new rectangle. In this way, an application can control the alignment of the preserved image as well, by changing the origin of the first rectangle.
If no change is made to either rectangle, the entire window area is preserved. If either rectangle is empty, the entire window area is completely redrawn by the operation.
Note: This functionality can be used to optimize window updating when the window is resized. For example, if the application returns that the window is to be aligned with the top-left corner, and the top border is sized, the screen data of the window moves with the top border.
In all instances, the rectangles are intersected with the area of the screen that is actually visible and the valid area of the window. That is, only the window area that contains window information is copied.
For example, consider an application that has two scroll bars, that are children of the client window. When the window is resized, the scroll bars must be completely redrawn. By returning rectangles that exclude the scroll bars, the area of the scroll bars is completely redrawn, thereby preserving only the part of the screen that is worth preserving.