Inhibit sleep with chromium/firefox while playing videos

I have a question, in Windows whenever a video would stop/end in Firefox/Chromium etc, after the defined time the screen would turn off and the PC would lock. This does not happen with KDE unfortunatelly. Whenever I leave my PC idle, it works perfectly fine but whenever I'm playing a video and it stops/ends, it doesn't. I tried blocking the apps but now my screen turns off even though a video is playing. Anyone have any tips? Anyone know how can I fix this?