Mac screensaver won’t quit, and legacyScreenSaver eats CPU

With a third-party screensaver installed, Activity Monitor may show a process called legacyScreenSaver still using CPU long after the screensaver left the screen. You’re not imagining it, and it isn’t a sign of malware. It’s a known problem with how macOS hosts screensavers that weren’t made by Apple.

What people actually see

In September 2023, on the first release of macOS 14 Sonoma, a screensaver developer described it on Apple’s developer forums: once the screensaver is dismissed, it keeps animating invisibly and keeps using CPU and GPU. Each new activation seemed to add another copy, and the screensaver view was never told to stop. The same code behaved on macOS Ventura. The thread cites the bug report number FB13041503.

Later replies in that thread describe the same thing on macOS 14.1.2 — roughly 20% CPU in the background on a three-display setup — and again in early 2024. A separate thread on Apple’s discussion boards, opened in November 2023 on macOS 14.1, asks why the process was holding 1.3 GB of memory.

What Apple has said

An Apple developer-support engineer replied in October 2023, asking whether it still reproduced on a newer macOS 14.1 beta and noting he had no solid evidence it was fixed there. In July 2024 he agreed that there are many bugs with third-party screensavers under Sonoma, and added that the old screensaver API “relies on a complicated compatibility shim, and that’s a source of ongoing problems.” His advice to developers: file an enhancement request for a new screensaver API built on app extensions.

The thread has no official fix, and none turned up in other sources either. Whether it still happens on a given Mac depends on the screensaver and the macOS release.

What you can try

None of these is an Apple-approved fix. They’re the practical options.

1. Confirm it’s the screensaver. Open Activity Monitor and look for legacyScreenSaver. If it keeps using CPU after you’ve woken the Mac, that’s the case described above.
2. Quit that one process. Select it in Activity Monitor and quit it. On Apple’s discussion boards, users report CPU dropping back to normal, and one of them says the screensaver and hot corners kept working. That’s anecdotal, not Apple guidance.
3. Update the screensaver. Some authors ship their own workaround. Release 1.1.5 of the free Matrix.saver, for example, notes a workaround for this Apple bug, which it describes as increasing CPU and memory use every time the screensaver is invoked.
4. Remove third-party screensavers you don’t use, then restart. With none installed, there’s nothing for that process to host.
5. Use something that isn’t a screensaver. A screensaver runs inside that hosting process. A regular app doesn’t, so the bug doesn’t reach it.

Where RainGlyph fits

RainGlyph is a digital-rain app for macOS. It’s an ordinary menu-bar app that draws the rain behind your desktop icons — not a .saver bundle — so it doesn’t run inside legacyScreenSaver. That only covers RainGlyph itself: any other third-party screensaver you keep installed can still trigger the behaviour above.

If what you’re after is just the Matrix effect, the guide to every real option on a Mac compares them, free ones included.

FAQ

Is it safe to quit legacyScreenSaver in Activity Monitor?

Users on Apple’s discussion boards report that force-quitting it brings CPU back to normal, and one of them says the screensaver and hot corners kept working afterwards. Treat this as user reports, not Apple guidance.

Does this affect Apple’s own screensavers?

The reports cited here all concern third-party screensavers.

Will Apple fix it?

Nothing public says so. The one recommendation from Apple’s side was for developers to file an enhancement request for a new screensaver API.

Sources: Apple Developer Forums thread 738547 (September 2023 – July 2024); Apple Support Community thread 255256761 (November 2023 – October 2024); Matrix.saver release notes on GitHub.