WebVideoFullscreenInterfaceAVKit and WebVideoFullscreenModelMediaElement are used in both WK1 and WK2. In WK2 calls into them happen in the correct thread automatically. In WK1 they call directly into each other and so dispatching to the correct thread is required. This should be refactored so that WebThreadRun is only used in the WK1 implementation.
This has been fixed by other changes.