Visual indication of an expired login session

When you leave page open in the browser and you come back some time later - after your login session expired - you don’t have any indication that it expired. When you do some changes then, for example in Settings, you will lose all the changes because you’ll be redirected to login page and the changes won’t be saved obviously.

There should be some periodic check as is on other services to see if you are online and can continue editing and if not, warning should be shown. And/or taking you back from the login screen to where you were with all the values set before the save button was pressed, but that seems like more complex solution.