In earlier Ubuntu versions like 16.04 and 17.04 while watching YouTube videos in full-screen mode, the screen would not get locked automatically.
In Ubuntu 17.10 while watching videos in browser, system is getting locked. So Ubuntu 17.10 is considering this scenario as user is not doing anything.
Kindly suggest a solution for that.
The reason that this does not work any more is because 17.10 is the first ubuntu release that uses wayland as its display server. It looks like preventing screen lock is an entirely missing feature in Xwayland.
It has been filed as a bug in Ubuntu and hopefully will be fixed soon. If you're affected by this bug, please click the launchpad link below and click "Does this bug affect you?" to increase the bug priority.
As a workaround, you could also select "GNOME on X.org" as your session type when logging on.
There is also a bug in chromium, but it seems to apply only for google hangouts sessions: