Screenshots blank when taken in post-processing example

[jywarren]

First, AMAZING work especially on postprocessing in 1.7.0 - huge fan and so grateful.

Screenshots come out blank when I use this code, and I was able to confirm on the postprocessing example: https://aframe.io/aframe/examples/showcase/post-processing/ that this happens there too.

document.querySelector('a-scene').components.screenshot.capture('equirectangular');

(using code from https://aframe.io/docs/1.7.0/components/screenshot.html#methods)

The following image results (and same for perspective screenshots):

Is there an order of render change or something to do with postprocessing? The other showcase examples (in the 1.7.0 blog post) do not have this issue; they render normally.

Thank you!!