External monitor won't display a browser window in full screen—it jumps back to MacBook Pro screen

Watching a live video in a browser window on an external HDMI monitor. Works fine. But if I click the "zoom to full screen" button in the browser window, it goes to full screen—but on my MacBook Pro's screen. I can't get full screen on the external monitor.

This doesn't happen with all feeds, but does with most of them.