[Bug]: Menu bar icon disappears on large multi account file sync in macOS

### ⚠️ Before submitting, please verify the following: ⚠️

- [x] This is a **bug**, not a question or a configuration issue.
- [x] This issue is **not** already reported on Github (I've searched it).
- [x] Nextcloud Server and Desktop Client are **up to date**. See [Server Maintenance and Release Schedule](https://github.com/nextcloud/server/wiki/Maintenance-and-Release-Schedule) and [Desktop Releases](https://nextcloud.com/install/#install-clients) for supported versions.
- [x] I agree to follow Nextcloud's [Code of Conduct](https://nextcloud.com/contribute/code-of-conduct/)

### Bug description

Hello,

I have been experiencing consistent crashes for the Nextcloud macOS client over the last week when trying to synchronise a large number of files (200,000 photos) to my server. I have been stumped as to why and resorted to asking Claude to see if he can figure out what I am doing wrong. It looked at the diag files I am getting and the logs and suggested some bug I do not fully understand, but maybe you can find it helpful. Feel free to ignore, as it is AI-assisted, but the crashes are real, and I see them. As far as I can tell, the diag and log samples don't contain my private info, so I hope these help, and I hope Claude's thing was onto something real, not fake. ;-)

Cheers,
Alex.

--- CLAUDE FROM HERE ---

Bug description

The macOS GUI process spends ~96% of sampled time in a tight loop polling AppleInterfaceStyle (dark/light mode preference) via CFPrefsPlistSource, combined with QSettings::sync() calls from QtNetwork that take a QLockFile and write to disk. This saturates the main UI thread, dirties ~8.6 GB of file-backed memory in 11 hours, and triggers macOS resource-budget diagnostic reports.

User-visible symptoms include:

* The menu bar icon becomes unresponsive or disappears entirely while File Provider extensions continue syncing in the background
* Activity panel ceases updating
* App appears to "crash" but is actually thrashing — killall Nextcloud and relaunch is the only recovery
* macOS files Nextcloud_*_kanga.diag reports under /Library/Logs/DiagnosticReports/ for excessive disk writes
* Footprint grows from ~67 MB at launch to >1.6 GB within ~14 minutes of sampling

This may be the underlying cause of several existing reports, including #9137, #7377, #7692, #6139, and #5121. Filing separately because none of those identifies the AppleInterfaceStyle + QSettings::sync pattern as the root cause.

### Steps to reproduce

* Configure Nextcloud Desktop with multiple accounts (in my case, 5).
* Enable File Provider mode (macOS-native virtual files).
* Begin initial sync of a large library (in my case, ~213k photos on the server side).
* Observe the GUI within 10 minutes.

IMPORTANT: My workaround is to disable sync for all 5, then enable it on JUST ONE. It seems to run for a little longer, but eventually, after 20-30 minutes, it crashes. :-)

BUT, only the GUI process crashes; these stay:

% ps aux | grep -i nextcloud | grep -v grep | head -5
alex 18875 0.0 0.1 442672576 35152 ?? Ss Thu07PM 1:08.44 /Applications/Nextcloud.app/Contents/PlugIns/FileProviderExt.appex/Contents/MacOS/FileProviderExt -LaunchArguments eyJ0eXBlIjoxLCJlbmhhbmNlZFNlY3VyaXR5IjpmYWxzZSwic2VydmljZU5hbWUiOiJjb20ubmV4dGNsb3VkLmRlc2t0b3BjbGllbnQuRmlsZVByb3ZpZGVyRXh0In0=
alex 18874 0.0 0.1 442657552 35152 ?? Ss Thu07PM 1:35.84 /Applications/Nextcloud.app/Contents/PlugIns/FileProviderExt.appex/Contents/MacOS/FileProviderExt -LaunchArguments eyJ0eXBlIjoxLCJlbmhhbmNlZFNlY3VyaXR5IjpmYWxzZSwic2VydmljZU5hbWUiOiJjb20ubmV4dGNsb3VkLmRlc2t0b3BjbGllbnQuRmlsZVByb3ZpZGVyRXh0In0=
alex 18873 0.0 0.1 442677952 35184 ?? Ss Thu07PM 1:07.06 /Applications/Nextcloud.app/Contents/PlugIns/FileProviderExt.appex/Contents/MacOS/FileProviderExt -LaunchArguments eyJ0eXBlIjoxLCJlbmhhbmNlZFNlY3VyaXR5IjpmYWxzZSwic2VydmljZU5hbWUiOiJjb20ubmV4dGNsb3VkLmRlc2t0b3BjbGllbnQuRmlsZVByb3ZpZGVyRXh0In0=
alex 18872 0.0 0.1 442874544 39008 ?? Ss Thu07PM 1:06.94 /Applications/Nextcloud.app/Contents/PlugIns/FileProviderExt.appex/Contents/MacOS/FileProviderExt -LaunchArguments eyJ0eXBlIjoxLCJlbmhhbmNlZFNlY3VyaXR5IjpmYWxzZSwic2VydmljZU5hbWUiOiJjb20ubmV4dGNsb3VkLmRlc2t0b3BjbGllbnQuRmlsZVByb3ZpZGVyRXh0In0=
alex 18871 0.0 0.1 443021264 45488 ?? Ss Thu07PM 2:12.27 /Applications/Nextcloud.app/Contents/PlugIns/FileProviderExt.appex/Contents/MacOS/FileProviderExt -LaunchArguments eyJ0eXBlIjoxLCJlbmhhbmNlZFNlY3VyaXR5IjpmYWxzZSwic2VydmljZU5hbWUiOiJjb20ubmV4dGNsb3VkLmRlc2t0b3BjbGllbnQuRmlsZVByb3ZpZGVyRXh0In0=
alex@MacBookPro ~ %

### Expected behavior

--- AGAIN FROM CLAUDE THINGAMEJIG ---

The GUI should remain responsive, the menu bar icon should stay visible, and the app should not consume resources at a rate that triggers macOS resource-budget warnings.

The AppleInterfaceStyle preference should be observed via NSDistributedNotificationCenter's AppleInterfaceThemeChangedNotification rather than polled. QSettings::sync() should not run on the main thread on every QtNetwork operation completion.

Actual behavior

From log stream --predicate 'process == "Nextcloud"' --info --debug (sample attached)

The Nextcloud process emits this pair of messages 30-40 times per second, indefinitely:

```
Nextcloud: (CoreFoundation) [com.apple.defaults:User Defaults] looked up value for key AppleInterfaceStyle in CFPrefsPlistSource<...> (Domain: kCFPreferencesAnyApplication, User: kCFPreferencesCurrentUser, ByHost: No, Container: (null), Contents Need Refresh: No) via CFPrefsSearchListSource<...> (Domain: com.nextcloud.desktopclient, Container: (null) Non-launch persona: 0)
Nextcloud: (FrontBoardServices) [com.apple.FrontBoard:SceneClient] [com.apple.controlcenter:...] Sending action(s) in update: NSSceneFenceAction
```

The NSSceneFenceAction between each query indicates a UI redraw cycle is being forced on each poll.

From Nextcloud_2026-05-01-144224_kanga.diag (attached)

macOS resource-budget diagnostic report:

```
Event: disk writes
Writes: 8589.94 MB of file backed memory dirtied over 39317 seconds
(218.48 KB per second average), exceeding limit of 99.42 KB per second
over 86400 seconds
Footprint: 67.44 MB -> 1640.97 MB (+1573.53 MB)
```

Heaviest stack (201/209 samples) for the main thread:

```
QCoreApplication::exec()
→ QEventLoop::exec(...)
→ libqcocoa.dylib (NSApplication event loop)
→ QApplicationPrivate::notify_helper(...)
→ QObject::event(...)
→ QtNetwork (×90 samples, multiple offsets)
→ QSettings::sync()
→ QLockFile::tryLock(...)
→ write() syscall
```

This indicates that QtNetwork is invoking QSettings::sync() on completion of network operations, on the main thread, which acquires a lock file and performs a synchronous write repeatedly, for every network operation.

Server configuration
(Omitted — issue is purely client-side and may reproduce against any server.)

Client configuration

Nextcloud Desktop: 33.0.3 (33.0.3.0)
Operating system: macOS 26.4.1 (Build 25E253)
Hardware: MacBook Pro 18,4 (M1 Max), 64 GB RAM, arm64
Installation: Direct download (Team ID: NKUJUXUJ3B, signed by Nextcloud GmbH)
Mode: File Provider (macOS-native virtual files)
Accounts configured: 5 (one primary, four secondary all paused in GUI)
File Provider extensions running: 4 (one per non-primary account, persistent across GUI restarts)

[nextcloud-diag.zip](https://github.com/user-attachments/files/27297746/nextcloud-diag.zip)
[nextcloud-log-sample.txt](https://github.com/user-attachments/files/27297745/nextcloud-log-sample.txt)

Logs

Two files attached:

nextcloud-diag.zip — full .diag report from /Library/Logs/DiagnosticReports/
nextcloud-log-sample.txt — ~10 seconds of log stream output showing the polling loop

Additional context — possible related issues

#9137 — Menu bar icon no longer shows sync status (likely surface symptom of GUI being unresponsive)
#9724 — Memory leak / infinite loop pattern (different specific trigger, similar shape)
#7377 — macOS becomes unresponsive at 100% CPU (likely same root cause)
#7692 — Status icons missing when other File Providers active (related: this report has 4 File Provider extensions active)
#6139 — No icon in menu bar with extensions still running (matches my "icon disappears" symptom)
#5121 — Menu unresponsive, only killall Nextcloud recovers (matches recovery pattern)

Suggested investigation areas

1. Theme change detection — replace any polling-based reads of AppleInterfaceStyle with subscription to NSDistributedNotificationCenter's AppleInterfaceThemeChangedNotification.

2. QtNetwork → QSettings interaction — investigate why network operation completion triggers QSettings::sync(). Likely candidate: cookie jar persistence, OAuth token storage, or QNetworkAccessManager configuration. Move to a worker thread or batch writes.

3. Multi-account scaling — the load is N× per active account. Investigate whether shared state can be centralized.

### Which files are affected by this bug

None

### Operating system

macOS

### Which version of the operating system you are running.

Tahoe 26.4.1

### Package

Official macOS 12+ universal pkg

### Nextcloud Server version

32.0.8.2

### Nextcloud Desktop Client version

33.0.3.0

### Is this bug present after an update or on a fresh install?

Updated from a minor version (ex. 3.16.1 to 3.16.2)

### Are you using the Nextcloud Server Encryption module?

Encryption is Enabled

### Are you using an external user-backend?

- [ ] Default internal user-backend
- [ ] LDAP/ Active Directory
- [ ] SSO - SAML
- [ ] Other

### Nextcloud Server logs

```shell

```

### Additional info

_No response_