Any way to detect what is disabling the screensaver?

Run powercfg /requests from an Admin Command Prompt. For example, mine printed:

DISPLAY:
[PROCESS] \Device\HarddiskVolume8\Games\Origin\Origin.exe
Playing video

SYSTEM:
[DRIVER] Realtek High Definition Audio (HDAUDIO\FUNC_01&VEN_10EC&DEV_0900&SUBSYS_10438560&REV_1000\4&9793a48&0&0001)
An audio stream is currently in use.

AWAYMODE:
None.

Which indicates that Origin was playing a video and this was preventing the screensaver from running.

powercfg is built into windows and lets you control and view lots of power management settings.

powercfg -energy -trace

This will create a very detailed power management event log, energy-trace.etl. Open it with Event Viewer and filter log by Event Source: Kernel-Power, Include event ID: 62.

This should give you events that look like:

The application or service \Device\HarddiskVolume1\games\Steam\Steam.exe has overridden user power management settings with a code of 0x3.

The code may be different depending whether screensaver, monitor sleep, or another state transition was prevented.

Sometimes just changing the time on the screensaver (not the type) to something else, say from 10 min down to 1 and then back to what you had it before will allow the screen saver to function normally.

Thanks to W1N9Zr0 for pointing me in the right direction!

In my case, it was an XP Pro (SP3) machine so the "energy" switch was not available, However, the "devicequery" switch indicated that it was either the mouse or the keyboard. Once I switched from the PS/2 keyboard I was using to a USB keyboard, the screensaver (and other power options) kicked in per the settings.

NOTE: When screensaver was set to 1 minute, it would sometimes kick in so I knew it was working. Anything above 1 minute and the screensaver would never kick in.

I find the culprit is typically a media player. You'd think it'd only be when playing video, but I have had some music players hold off the screen saver. For example I had Amazon Music and Groove open and paused today, and just found my screen still unlocked after an hour.

I'm not sure which is to blame, but I suspect them over the IDE and browser I was running, both of which do let the screen lock.

For a Mac you can try opening the Activity Monitor going to the Energy Tab and ordering fields by Preventing Sleep. You should be able to easily identify the guilty application ✅

I have struggled with this for years, there always seems to be something that wants to prevent your screen going blank, and give your screen burn in. This will iterate through all the items preventing your screen from going blank, and disable them, then force quit anydesk instances as active sessions in anydesk prevent a monitor turning off. Then it executes a command to shut the monitor off via powershell.

@echo off

>nul 2>&1 "%SYSTEMROOT%\system32\cacls.exe" "%SYSTEMROOT%\system32\config\system"
if '%errorlevel%' NEQ '0' (
    echo Requesting administrative privileges...
    powershell.exe -Command "Start-Process -FilePath '%~f0' -Verb RunAs"
    exit /b
)

setlocal EnableDelayedExpansion
echo Finding all active display requests...

set "proclist="
set "inDisplaySection=false"
for /f "delims=" %%a in ('powercfg /requests') do (
    set "line=%%a"

    rem Section headers in powercfg output don't have leading spaces and end with a colon.
    if /i "!line!" == "DISPLAY:" (
        set "inDisplaySection=true"
    ) else if "!line:~-1!" == ":" (
        set "inDisplaySection=false"
    )

    if "!inDisplaySection!" == "true" (
        rem Check if the line (which might be indented) contains [PROCESS]
        echo !line! | findstr "[PROCESS]" >nul
        if !errorlevel! == 0 (
            rem Split the line. %%b will be "[PROCESS]", %%c will be the path.
            for /f "tokens=1,*" %%b in ("!line!") do (
                set "procpath=%%c"
                rem This for loop is a trick to get filename.ext from a full path.
                for %%d in (!procpath!) do (
                    set "procname=%%~nxd"
                    set "proclist=!proclist! "!procname!""
                )
            )
        )
    )
)

if defined proclist (
    echo Overriding display requests for the following processes:!proclist!
    for %%p in (!proclist!) do (
        echo Overriding display request for %%~p
        powercfg -requestsoverride PROCESS "%%~p" display system
    )
) else (
    echo No active display requests found to override.
)
echo Done overriding.

endlocal

setlocal enabledelayedexpansion
echo.
echo Restarting Anydesk processes to clear potential display locks...
set "anydesk_paths="
for /f "skip=1 delims=" %%a in ('wmic process where "name like 'anydesk%%' and ExecutablePath is not null" get ExecutablePath') do (
    set "path=%%a"
    for /f "tokens=*" %%b in ("!path!") do set "path=%%b"
    if defined path (
        rem Check if the path is already in the list to avoid duplicates.
        echo "!anydesk_paths!" | findstr /C:"\"!path!\"" >nul
        if !errorlevel! neq 0 (
            set "anydesk_paths=!anydesk_paths! "!path!""
        )
    )
)

if defined anydesk_paths (
    echo Found running Anydesk processes. Terminating them...
    taskkill /f /im anydesk*.exe /t >nul 2>&1
    
    echo Waiting for processes to close...
    timeout /t 1 /nobreak >nul

    echo Relaunching Anydesk applications...
    for %%p in (!anydesk_paths!) do (
        if exist %%~p (
            echo   - Starting %%~p
            start "" "%%~p"
        ) else (
            echo   - Path not found, cannot restart: %%~p
        )
    )
) else (
    echo No running Anydesk processes found.
)
endlocal

start "" cmd /c "powershell.exe -Command "(Add-Type '[DllImport(\"user32.dll\")]public static extern int SendMessage(int hWnd,int hMsg,int wParam,int lParam);' -Name a -Pas)::SendMessage(-1,0x0112,0xF170,2)""

I don't know of any program that does this, but the way to find out is via the GetThreadExecutionState() function which will hold the value ES_DISPLAY_REQUIRED if the thread is currently blocking the screensaver from being activated.

If you apply this function to all of the threads in the system and then call GetProcessIdOfThread on the thread that you find, you'll have found the process that is blocking the screensaver.