UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.133 Safari/537.36

Steps to reproduce the problem:
1. Open a window that produces a child window (e.g. https://www.w3schools.com/tags/tryit.asp?filename=tryhtml_select)
2. Use desktopCapture/getUserMedia to capture the window (not the entire screen)
3. Toggle the select-input to display options.

What is the expected behavior?
The options are rendered as part of the current stream

What went wrong?
No change to the current stream

Did this work before? No 

Does this work in other browsers? No
 FireFox: Same behavior

Chrome version: 57.0.2987.133  Channel: n/a
OS Version: OS X 10.12.4
Flash Version: 

I have tested this on MacOS 10.12.4 and Windows 10.

As far as I can tell, part of the reason is that streams are bound to a specific window ID and in both MacOS and Windows, child windows get their own unique id, only sharing process/pid with its parent. 
I understand that this can be perceived as intended behaviour but as a user, I expect contextual items like drop-downs on a webpage or popovers/toolbars in a native window are part of the source I choose to share.

When sharing the whole screen, things are working as expected.