Is it possible that you do the caching direct in WIDGET? if so, your code should work well. Inside the paint event check if your pixmap is still valid. If not, update it. otherwise just paint it. Like you do in your cached variant.

Why render is sending an update is probably that it want ensure that the right/updated view is rendered.


And I see right now: Shouldn't it be: WIDGET::render(&m_cache)?