So logical to "unseparate" these features ... and also that the prevent locking hot corner doesn't do anything ...

If the prevent screen locking corner doesn't do anything (including preventing the screen from being locked) then that would be a bug in KWin. If it prevents the screen from being locked, but the screensaver is still enabled, then there isn't much that can be done i'm afraid.

it is a known bug:
https://bugs.kde.org/show_bug.cgi?id=315116

please vote and/or share your experiences. I can't believe we are the only ones watching youtube and using screenlock/screensaver?

I have set my screensaver to power-off the display after 5 minutes. It often kicks in when I watch video with browser-based or locally installed players. I even had a shell script to prevent it, but after some time I got too lazy to start it.

I was just doing a web search again to find a cure, which lead me to this forum thread. Here are further search results:

- https://bugs.kde.org/show_bug.cgi?id=292917 VLC can't block KDE screensaver
- https://silviumc.wordpress.com/2011/12/ ... b-browser/ How to temporarily disable the screen saver in KDE while watching long Flash clips in a web-browser
- http://lists.kde.org/?l=kde-linux&m=123386866021638 Re: [kde-linux] Why is it so difficult to disable a screensaver?
- http://www.kde-forum.org/artikel/17259/ ... -open.html [kaffeine] Screensaver starts when I watch a video [open]

So, the problem is old and you are not alone with the issue.

About the screen edge screen locking prevention:
I tried it (KDE Development Platform: 4.14.3) and set "Prevent Screen locking" to the lower-right corner. I have not waited for the screensaver yet but noticed the as soon as the mouse is in the corner, the corner gets highlighted which overlaps with a fullscreen movie which I find distracting. I have not found an option to disable the highlighting. On the other hand, it makes sense to do some highlighting to know that an edge is "active".

So, what about not using screen edges but having a new service in the KDE "Service Manager" that watches for known video applications and/or fullscreen applications. Or maybe extend an existing one (there are "KScreen 2 - Screen management" and "Power managment" which sound related)