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Son Tran (Snow)
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Safari quits in the background: investigation log

Safari disappears when another app is focused; clicking the Dock icon restores the tabs. Cause not found. This log records what was observed, what the docs say, and what stays unverified.

Symptom

  • Safari quits while another app has focus; tabs restore on relaunch
  • Seen on macOS 15.7.7 (stable) and macOS 27.0 beta (build 26A5368g)
  • Started about 2 months before the log, with no system or Safari update in that window
  • Low Power Mode off (pmset -g | grep lowpowermode → 0)

Signals found

Three different reports, not necessarily one cause.

ReportWhereContent
Crash report, Jun 17 (15.7.7)SafariPAC_EXCEPTION in the scroll momentum thread
.diag, Jun 30 (27.0 beta)/Library/Logs/DiagnosticReportscom.apple.WebKit.Networking: 2147.49 MB written over 14694 s (about 146 KB/s) against a limit of 24.86 KB/s
Consolelaunchservicesdsandbox denial
  • .diag stack: SQLiteStorageArea::commitTransactionIfNecessary → SQLiteTransaction::commit → sqlite3_step
  • fs_usage showed writes to AlternativeService.sqlite and ServiceWorkerRegistrations-8.sqlite3
  • lsof at the time showed no abnormal Safari storage
  • The 24.86 KB/s limit equals 2 GB / 86400 s

What the docs say

  • Apple's EXC_RESOURCE documentation: a report marked "NON-FATAL CONDITION" is generated without terminating the process; subtype IO means excessive disk writes
  • Apple DTS on a disk-writes report (forum thread 730744, iOS): "That's not a crash"; Action taken: none means the system did not terminate the process
  • So the .diag alone does not show that Safari was killed

Ruled out or not applicable

  • Beta-only bug: symptom also on stable 15.7.7
  • Low Power Mode: off
  • Recent update: none in the 2 months before

Unverified

  • Action taken field of the .diag was never read
  • Whether the disk-write report and the quit are related at all (report may be log-only)
  • Which tab or site produced the writes: never identified
  • Whether the scroll PAC_EXCEPTION crash and the background quit share a cause
  • Third-party claims that the 24 h disk-write budget sends SIGKILL: source not reachable, unconfirmed

Checks that would settle it

  • Read Action taken in the .diag
  • Safari with a blank page, another app focused for about 5 minutes: if it survives, a specific site is involved; if it still quits, the cause is Safari itself
  • Compare quit timestamps against .diag and crash report timestamps