The runtime effect vertex shader provides the fragment with the local coordinates as a varying. Until Vulkan 1.3 or khr_maintaince_4, its an error for the fragment shader to not consume this varying. As a result of this, any runtime_effect contents that do not use FlutterFragCoord will cause a validation error (and are technically invalid).

We could optionally enable khr_maintaince_4, but some devices may not support this.