Redrawing the screen when the window moves

I'm sure that this is an elementary operation: I am implementing a simple line drawing program and currently when I drag the drawing window up and down I lose what is on it. Does anyone have a suggestion on how to redraw the window after it is moved? Thank you very much.

if you are using glut just put a glutPostRedisplay at the last part of your reshape func.

if you are using something else put the appropriate redisplay function at the bottom of your reshape (or window moved) functions