We want to be able to experiment with this alongside other MR features, so it ought to be behind Nimbus rather than just a pref.

https://hg.mozilla.org/mozilla-central/rev/56c6e1791c69

Backed out for causing failures on browser_startup_mainthreadio.js

• backout: https://hg.mozilla.org/integration/autoland/rev/77ed2141a235a3656832f75c4345e56fa18877e8
• push: https://treeherder.mozilla.org/jobs?repo=autoland&revision=56c6e1791c69370a521249b58ec6ba26d8e1960d&group_state=expanded&selectedTaskRun=Up9OZyW8SUaY7aBzAB9H0g.0
• failure log: https://treeherder.mozilla.org/logviewer?job_id=388332090&repo=autoland&lineNumber=4167

[task 2022-08-24T09:40:34.436Z] 09:40:34     INFO - (PoisonIOInterposer) write - C:\Users\task_166132656680949\AppData\Local\Temp\tmpp8d2sftm.mozrunner\places.sqlite-wal
[task 2022-08-24T09:40:34.437Z] 09:40:34     INFO - Buffered messages finished
[task 2022-08-24T09:40:34.437Z] 09:40:34     INFO - TEST-UNEXPECTED-FAIL | browser/base/content/test/performance/browser_startup_mainthreadio.js | unexpected read on C:\Users\task_166132656680949\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Nightly Private Browsing.lnk before becoming idle - 
[task 2022-08-24T09:40:34.437Z] 09:40:34     INFO - Stack trace:
[task 2022-08-24T09:40:34.439Z] 09:40:34     INFO -   0x7ffefb09356d
[task 2022-08-24T09:40:34.439Z] 09:40:34     INFO -   nsIWindowsShellService.hasMatchingShortcut
[task 2022-08-24T09:40:34.439Z] 09:40:34     INFO -   task (resource:///modules/BrowserGlue.jsm:2550:20)
[task 2022-08-24T09:40:34.439Z] 09:40:34     INFO -   _scheduleStartupIdleTasks/< (resource:///modules/BrowserGlue.jsm:2888:8)
[task 2022-08-24T09:40:34.439Z] 09:40:34     INFO -   ChromeUtils::IdleDispatch handler
[task 2022-08-24T09:40:34.439Z] 09:40:34     INFO -   Task ChromeUtils::IdleDispatch
[task 2022-08-24T09:40:34.439Z] 09:40:34     INFO -   XREMain::XRE_main
[task 2022-08-24T09:40:34.439Z] 09:40:34     INFO - (PoisonIOInterposer) read - C:\Users\task_166132656680949\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Nightly Private Browsing.lnk
[task 2022-08-24T09:40:34.439Z] 09:40:34     INFO - Not taking screenshot here: see the one that was previously logged
[task 2022-08-24T09:40:34.440Z] 09:40:34     INFO - TEST-UNEXPECTED-FAIL | browser/base/content/test/performance/browser_startup_mainthreadio.js | unexpected stat on C:\Windows\SysWOW64\propsys.dll before becoming idle - 
[task 2022-08-24T09:40:34.440Z] 09:40:34     INFO - Stack trace:
[task 2022-08-24T09:40:34.440Z] 09:40:34     INFO -   0x7ffefb09356d
[task 2022-08-24T09:40:34.441Z] 09:40:34     INFO -   nsIWindowsShellService.hasMatchingShortcut
[task 2022-08-24T09:40:34.441Z] 09:40:34     INFO -   task (resource:///modules/BrowserGlue.jsm:2550:20)
[task 2022-08-24T09:40:34.441Z] 09:40:34     INFO -   _scheduleStartupIdleTasks/< (resource:///modules/BrowserGlue.jsm:2888:8)
[task 2022-08-24T09:40:34.441Z] 09:40:34     INFO -   ChromeUtils::IdleDispatch handler
[task 2022-08-24T09:40:34.441Z] 09:40:34     INFO -   Task ChromeUtils::IdleDispatch
[task 2022-08-24T09:40:34.441Z] 09:40:34     INFO -   XREMain::XRE_main
[task 2022-08-24T09:40:34.441Z] 09:40:34     INFO - (PoisonIOInterposer) stat - C:\Windows\SysWOW64\propsys.dll
[task 2022-08-24T09:40:34.441Z] 09:40:34     INFO - Not taking screenshot here: see the one that was previously logged

Looks like this main thread I/O caused by the expected (one time) shortcut creation that we added to fix https://bugzilla.mozilla.org/show_bug.cgi?id=1762994. We're either going to need to add an exception for it (tempting, as it's a one time only thing, and should only be happening for zip builds), or move it off main thread.

Backout merged to central: https://hg.mozilla.org/mozilla-central/rev/77ed2141a235a3656832f75c4345e56fa18877e8

This happens during lazy startup, and we try to avoid any main thread I/O there. Even though this will only ever happen once, there's really no reason I know of to keep it on the main thread.

Because we're enabling window separation by default we also need to enable shortcut creation in the installer to minimize the possibility of users hitting https://bugzilla.mozilla.org/show_bug.cgi?id=1762994.

Backed out for causing Assertion failures on nsDirectoryService.cpp

• Backout link
• Push with failures
• Failure Log
• Failure line: Assertion failure: NS_IsMainThread() (Do not call dirsvc::get on non-main threads!), at /builds/worker/checkouts/gecko/xpcom/io/nsDirectoryService.cpp:191

Because we're enabling window separation by default we also need to enable shortcut creation in the installer to minimize the possibility of users hitting https://bugzilla.mozilla.org/show_bug.cgi?id=1762994.

Depends on D155598

I don't see any reason to keep this under the same namespace as the other former privacySegmentation prefs - but perhaps I'm missing something here.

Depends on D156733

https://hg.mozilla.org/mozilla-central/rev/7475c0da3352
https://hg.mozilla.org/mozilla-central/rev/b241dfa9a9c8
https://hg.mozilla.org/mozilla-central/rev/b4f418acd214
https://hg.mozilla.org/mozilla-central/rev/b1462bbc7934