Only One Window Visible at a Time on macOS 27 Golden Gate
This sounds like a window-management issue, possibly related to the macOS 27 upgrade.
Before assuming it's a bug, I'd check a few settings that can cause windows to disappear when switching between apps.
I would suggest trying the following steps in order:
1. Check Stage Manager
This is the first thing I'd check, as Stage Manager changes how macOS displays and groups application windows.
- Go to Apple menu >> System Settings >> Desktop & Dock.
- Find the Stage Manager setting and turn it off temporarily if it's enabled.
- Open Mail and Safari, arrange them side by side, and switch between them.
If the problem goes away, you've likely found the cause. If you prefer using Stage Manager, look for Show windows from an application and select All at Once instead of One at a Time.
2. Check whether the window is actually closing
The distinction matters because a window may be hidden, minimized, or moved to another desktop rather than closed.
- Open Mail and Safari and arrange them side by side.
- Switch between the two apps.
- If one disappears, press Command–Tab to return to it.
- Check the Dock to see whether the window has been minimized, and open Mission Control (usually F3) to see whether it has moved to another desktop or Space.
If the window returns with its original contents, it probably wasn't actually closed.
3. Check your desktop and window settings
While in System Settings >> Desktop & Dock, check the options related to clicking the desktop, window management, and Mission Control. The exact wording may vary.
Also, make sure neither app is in full-screen mode. Try positioning the windows manually rather than using the green window button to tile them. If manual positioning works, the problem may be related to macOS's window-tiling behaviour.
4. Restart and check for updates
If the settings look correct, restart your iMac and check System Settings >> General >> Software Update for any available updates.
Since this may have started after upgrading to macOS 27, it's worth checking whether a subsequent update addresses a window-management issue.
5. See whether it affects every app
Try a few combinations, such as Finder and Safari or Notes and Mail.
- If only one app is affected, focus on that app's settings and behaviour.
- If every app behaves the same way, a system-wide setting or macOS issue is more likely.
If necessary, testing in a new macOS user account can help determine whether the problem is specific to your usual account.
If the problem persists
I'd avoid deleting preference files or reinstalling macOS until the simpler checks have been exhausted. If the issue affects all applications despite Stage Manager being off, it may be a macOS bug worth reporting to Apple Support.
One question that would help pinpoint the cause: When you switch apps, does the other window disappear but return when you click its Dock icon, or does it actually close so that you have to reopen the window?
That distinction, along with whether Stage Manager is enabled, should help narrow this down considerably.
For reference, Apple's guides cover Stage Manager, Desktop & Dock settings, and arranging app windows.
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