Prevent display from turning off due to inactivity when watching full screen video (windows 7)

But there is a workaround:

https://mousejiggler.codeplex.com/

Make sure you use the Zen Jiggle mode to avoid any onscreen interference with your viewing pleasure.

Actually the software CAN ask windows to turn power saving off. Thats how media players do it. But when you watch youtube or other flash videos it is the browser that's running, nothing else. There is a flash plugin but this is a plugin, it only has access to the same things the browser has. If the browser does not provide access to power saving management the plugin can't access it. So the only way for that to be solved would be that browsers could have a new feature that would disable power saving when you visit some certain sites or has the ability to manage power saving and somehow passes that access to third party plugins. I don't see there is another way. or, you could go to the start menu and click on power options (search it) and then once your in you can click on a thing that let's you change how long until the computer goes into sleep mode.

Maybe it depends on the browser and the operating system, but I can tell you that using Firefox 32.0.1 in Windows 7 64-bit, whenever a sound is played by the Flash plugin, it prevents the display from turning off. This can be seen by running powercfg -requests while a sound is playing (run cmd.exe as an administrator before typing the command):

C:\Windows\system32>powercfg -requests
DISPLAY:
[PROCESS] \Device\HarddiskVolume1\Program Files (x86)\Mozilla Firefox\firefox.exe

SYSTEM:
[DRIVER] IDT High Definition Audio CODEC (HDAUDIO\FUNC_01&VEN_111D&DEV_76DF&SUBSYS_1028053F&REV_1002\4&313bea4d&0&0001)
An audio stream is currently in use.

AWAYMODE:
None.

Two things are shown in this example output. It's the firefox.exe process under DISPLAY: that is preventing the display from turning off. The sound playing through the audio driver prevents the system from going to sleep.

I consider Firefox's behavior undesirable, as I seldom watch long YouTube videos, and I don't want every message notification from Facebook Chat to turn my screen on. The behavior can be changed by using a powercfg -REQUESTSOVERRIDE command, like this:

powercfg -REQUESTSOVERRIDE process "\Device\HarddiskVolume1\Program Files (x86)\Mozilla Firefox\firefox.exe" display

Type powercfg /? for help.