Background not re-generated when window is moved back onto screen

When a window is moved off the screen on Symbian, the backing store is destroyed to save memory. When the window is moved back on to the screen, the backing store is recreated and the entire window is invalidated so that it can repopulate the backing store with content. However, if only a portion of the window is moved on to the screen, this implies that only a portion of the background will be filled. Subsequent moves do not result in a paint events (as an optimization), but the exposing of the new area should generate paint events. The problem is that it currently doesn't because QSymbianControl::Draw() simply does a bit blit of the old backing store contents.

The reason that function does that instead of calling sync() (to update the backing store contents) is that the sync() code path will eventually call DrawNow() which will result in a call to Draw() so calling sync() would result in recursion. This causes additional problems because you cannot call Draw() recursively because it will cause multiple activations of the graphics context which results in a panic.