This sounds like your browser may be doing something weird to save computation. What browser and what platform are you on?
If you leave the tab open, it's supposed to continue to update. If the tab becomes hidden because you switch to another tab in the same brower window (and perhaps that's what you mean by it not having focus?) the display stops updating while the tab is hidden. However, when you switch back, the cameras are supposed to update again, possibly after a few seconds of delay to get everything going.
If you're leaving the tab visible, and switching to another browser _window_ it's not supposed to stop updating at all.
Assuming you are indeed switching tabs, are you waiting long enough to give the camera views a chance to update? In order to implement something that would tell you that the displays are not up to date, we need to first understand what's actually happening in your case. Each camera view already switches to displaying the last update time rather than saying "live" if the image being shown is more than several seconds old. What is shown in your case when the image is out of date?