I made some experiments and I understand one thing. Exaple two windows
/-------------------\ | W1 | | control1 | | | | /------------------------\ ------------- | | | | | | -----------> Move | W2 | | | ------------------------/
The window W1 is under window W2 When I move window W2 wine paint all rect of W1 window ! And it may hide control1 that inside area of W1. It is not good idea because not all rect of window need to repaint. Good idea is repaint only the part of W1 that under W2. Only rect that have any changes on screen. It will increase speed of drawing windows. But may be I mistake......