For display list based invalidation, we need to know the widget area to invalidate (generated from display list construction, layer construction, and ThebesLayer painting) before we trigger a widget paint event. We need to split layer transactions into a Validate() phase that draws all thebes layer contents, and then a separate Composite() phase. This is similar to what Shadow layers are doing ready. D3D10 layers already supports Validate() properly, the other backends could probably use better implementations that what I have here.

This is covered by the Great Layers Refactoring.