[UI] - Suspend/wake animations do not play when using physical power button via HandlePowerKey=suspend

[matte-schwartz]

Your system information

• Steam client version (build number or date): 1735842154
• Distribution (e.g. Ubuntu): CachyOS, SteamOS
• Opted into Steam client beta?: Yes
• Have you checked for system updates?: Yes
• Steam Logs: steam-logs.tar.gz - from CachyOS
• GPU: AMD Z1 Extreme (Phoenix APU)

Please describe your issue in as much detail as possible:

On non-Deck devices, unless a third-party tool like steam-powerbuttond is used, triggering suspend via the power button on other handhelds with HandlePowerKey=suspend results in no suspend or wake animation being played. Instead, the screen just fades to black on suspend and does the opposite while resuming. Triggering suspend via the left-side menu (Power -> Sleep) does show the animations properly, so the issue only occurs when using the physical power button to trigger suspend. This happens on both my ROG Ally running CachyOS, and my Legion Go running SteamOS 3.7.

Steps for reproducing this issue:

1. On a non-Deck handheld, load into gamescope-session
2. Trigger suspend with the power button (HandlePowerKey=suspend must be set)
3. Resume with the power button

You should note no animations play after 2 and 3, while they do on Steam Decks.

[matte-schwartz]

This may make more sense to put in SteamOS given that the Steam client does not monitor the power button, but since it also happens outside of SteamOS it's a bit of a gray area.