Unfortunately I couldn't reliably reproduce the issue, so I'm not 100% sure this fixes it. But I think there is a race condition that results in `updateCompositingRectSize` erroneously seeing the layer has no objects and skipping the update.
To address this, the compositing rect fill/size/pos are all now force-updated when the fill/objects are changed. Theoretically it should be impossible for the issue to occur now.