Thanks for your explanations Alex. Maybe useful for other: It turned out that Steam client has probably a bug preventing screensaver getting active. https://github.com/ValveSoftware/steam-for-linux/issues/5532 One more problem seems to be that Deepin or whatever still cant run the screensaver even after Steam was closed. https://github.com/linuxdeepin/developer-center/issues/567 Im sorry again for occupying this bug report.