WxPaintDC

From WxWiki

You may experience flickering in Scrolled Windows if, during the OnPaint/OnDraw, you select Bitmaps in a MemoryDC. (For example, at each call of OnPaint, you may want to create a MemoryDC, select an existing bitmap in it, and blit to the Scrolled Window DC.) To avoid this, you've got to create your Memory DC out of the onPaint/onDraw 'loop', just once.