WM_SIZE

This message is sent after the size of a window has changed.

Parameter Description  

wParam Contains a value that defines the type of resizing requested. It can be one of the following values:  
  Value Meaning
  SIZEFULLSCREEN Window has been maximized.
  SIZEICONIC Window has been minimized.
  SIZENORMAL Window has been resized, but neither SIZEICONIC nor SIZEFULLSCREEN applies.
  SIZEZOOMHIDE Message is sent to all pop-up windows when some other window is maximized.
  SIZEZOOMSHOW Message is sent to all pop-up windows when some other window has been restored to its former size.
lParam Contains the new width and height of the client area of the window. The width is in the low-order word; the height is in the high-order word.  

Comments

If the SetScrollPos or MoveWindow function is called for a child window as a result of the WM_SIZE message, the bRedraw parameter should be nonzero to cause the window to be repainted.