I had the same thing happen and at halftime figured out the solution. On the website on your laptop in the little Chromecast window at the top of the screen there is a drop down arrow. Open the drop down menu and change it to cast the tab rather than the video. Then it will cast the entire browser tab onto your TV, including the video. After that, set the video to full screen. That worked for me.
Theoretically I can then stream anything on any chrome web browser that way right?