UserAgent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:50.0) Gecko/20100101 Firefox/50.0

Steps to reproduce the problem:
Short version: try to render/screencast a child tab/window.
Example/Long version:
1. $ google-chrome-unstable --headless --remote-debugging-port=9222 https://francisduvivier.github.io/WebDriverTestHTML/blanktargettest.html
2. $ google-chrome-unstable http://localhost:9222/
3. Click to open the developer tools of the page
4. click the link in the page so a new tab is opened
5. $ google-chrome-unstable http://localhost:9222/
6. click to open the developer tools for the newly created page.
7. Try to screencast

What is the expected behavior?
rendered page is show in the developer tools for the new tab.

What went wrong?
Screencast for new tab/window not working "toggle screencast"-button does not do anything.

Did this work before? N/A 

Chrome version: 60.0.3080.5 (Official Build) dev (64-bit)  Channel: stable
OS Version: Mint 18.1 Cinnamon 64bit
Flash Version: 24.0.0.189 internal-not-yet-present

Similarly, when using chromedriver(2.29) with selenium in python and trying to get a screenshot after switching to a newly created window does not work, trying that blocks the thread indefinitely.

Tried this with chrome version 58 and version 60 on an intel and amd64-based system(both with linux mint 18.1 64 bit).

This leads me to believe that rendering for newly created tabs just does not work at all.