Chrome/Chrome OS Version: Whatever was running on the AOpen Chromebox Mini on December 28th, 2017.
Chrome OS Platform: AOpen Chromebox Mini
Network info: not sure, probably doesn't matter.

Please specify Cr-* of the system to which this bug/feature applies (add the label below). ← I don't see a button to do that anywhere.

Steps To Reproduce:
(1) Set up a ChromeOS device and a Chromecast on the same Wi-Fi network.
(2) Open a video on the ChromeOS from the file explorer.

Expected Result:
There is a cast button somewhere in the video app.

Actual Result:
The only way I can cast it to my TV is to put it full-screen and cast the entire desktop view. It's slow and I can't use my device while doing that.

How frequently does this problem reproduce? (Always, sometimes, hard to reproduce?)

Always.

What is the impact to the user, and is there a workaround? If so, what is it?

Chromecast isn't as useful as I originally thought. It's great for YouTube or other compatible apps, but there's no way to read local videos from the computer to the TV and use the computer at the same time?! =(