Screensaver activates behind non-Steam apps and flashes when using OSD

Your system information

• Steam client version: 1789086785
• SteamOS version: 3.9.0 20260828.100
• Opted into Steam client beta?: Yes
• Opted into SteamOS beta?: Yes
• Have you checked for updates in Settings > System?: Yes

Please describe your issue in as much detail as possible:

Expected:
When running a non-Steam application (e.g., Jellyfin, YouTube, EmulationStation, or any other flatpaks) in Gaming Mode the idle timer to prevent the screensaver from turning on should not be triggered.

Additionally, input or On-Screen Display function while a screensaver is active should cleanly wake the system and dismiss the screensaver state entirely before the OSD event fires.

Actual:
Non-Steam applications trigger the idle timer. As a result, the screensaver silently begins rendering in the background while actively playing or using a non-Steam app. Because non-Steam application windows are rendered on top of the Steam UI, the running screensaver remains hidden during normal gameplay.

Invoking any SteamOS function (e.g. adjusting volume) forces the underlying Steam UI to be rendered for the duration of the OSD toast.

This causes the active screensaver animation/logo (e.g., the bouncing logo screensaver or a fullscreen game hero art) to flash across the screen for 1 second every time an OSD action is triggered.

Said another way: watching a movie and then adjusting the volume causes a fullscreen game logo or the Steam DVD-bounce to briefly flash, which is both hilarious and disturbing.

Steps for reproducing this issue:

1. Enable the screensaver
2. Launch any non-Steam application in Gaming Mode
3. Keep the application running for longer than the screensaver timer.
4. With the non-Steam application open, trigger any system OSD overlay. I do this with a remote button bound to volume controls.
5. The screensaver graphic visually flashes on screen briefly. Actual function continues (volume changes, movie is playing uninterrupted)

Start by reproducing the issue in Gaming Mode with a non-Steam application, an enabled screensaver, and an OSD action after the idle timeout. Trace the screensaver idle handling and OSD wake/render sequence; done means non-Steam activity prevents activation and an OSD action does not flash the screensaver.