SC Tom wrote:
Win10Pro x64 v.1809 Build 17763.557
Acer V3-731 laptop

I watch a lot of tennis on-line since a lot of the good matches aren't
televised on my cable channels. It used to be, if I had a video running
full
screen (Firefox 67.0.3 (64-bit)), my screensaver would not start up and
interfere with it. Most of them use the Flash plug-in, and that also is the
latest and greatest version. Recently (within the last 3 or 4 days), the
screen saver starts up after the time set and continues to run until I move
my mouse or press a key, just as it should.

My desktop is one "update" behind (Build 17763.437), and the screen
saver on it
works as it should. Thinking the newer update (KB4503327) may be the
problem
(considering some of the other problems that have been posted about it), I
uninstalled it. No change- screen saver still starts during videos.

I could disable it, but that's not really fixing the problem. I seem to
remember
having this problem years (maybe even Windows versions) ago, but cannot
find
a fix for it. If anyone has had this problem and could provide a fix for
it, I would
be deeply appreciative. TIA!!

Do you have a password set for the screensaver ?

There is a claim in some MS documentation here, that the
behavior changed since Vista. If a password is set on the
screensaver, the screensaver can still switch on, even if
an application uses ES_CONTINUOUS = 0x80000000.

https://stackoverflow.com/questions/...-from-starting

https://stackoverflow.com/questions/...-from-starting

A second factor that is claimed to affect the screensaver
cutting in, is whether the video being viewed is "full screen"
or not. If the activity is "just a regular window", that can
allow the screensaver (with password?) to run.

Another thread, claims there can be an interaction with
added software. But that's the opposite direction, of never
getting the screensaver to work (when RGB software is installed
to modulate the fancy motherboard LEDs).

*******

powercfg /getactivescheme @ this returns the GUID of the current power schema

Power Scheme GUID: 381b4222-f694-41f0-9685-ff5bb260df2e (Balanced)

powercfg /query 381b4222-f694-41f0-9685-ff5bb260df2e # query call uses GUID

But the on-screen setting doesn't mention the screensaver
as such, it can be set to avoid "idling to sleep" and
the machine going into sleep state.

https://i.postimg.cc/LXpx12f5/idling-to-sleep.gif

*******

Summary: Think about what you added recently to the machine.
Or some policy change that inadvertently had this
as a side effect.

Paul