Found in

• Beta 105.0b2

Affected versions

• Beta 105.0b2

Tested platforms

• Affected platforms:
  Win 11 x64
• Unaffected platforms:
  Win 10 x64

Steps to reproduce

1. Download, unzip this build and open it.
2. Navigate to about:config and add the following prefs as true:
   browser.privacySegmentation.enabled
   browser.privacySegmentation.windowSeparation.enabled - this pref needs to be manually created
3. Update the build to the latest beta version available.
4. Close the build.
5. Open again the Firefox build, and observe the Win taskbar.

Expected result

• Firefox icon is the regulator one, not the PBM icon, as briefly is displayed in the taskbar.

Actual result

• Firefox PBM icon is briefly displayed in the taskbar, then it changes to the regular one.

Regression range

• Not a regression.

Additional notes

• Please observe the attached screencast.
• I was unable to reproduce the problem if updating from 104.0b1 to the latest version. So, maybe, this issue is triggered by the fact that we manually create the pref browser.privacySegmentation.windowSeparation.enabled in 99.0b1, before updating the build.

I'm going to take at least a quick look at this regardless, but is this reproducible by starting with a newer version? Eg: does it happen if you go from 105.0b1 -> 105.0b2?

I'm still curious what you find, but I think this is not related to the privacy segmentation prefs (although it's more serious when windowSeparation is true). I was able to reproduce what is effectively the same issue by following the same STR without setting the prefs. Instead of going from a Private Browsing icon to a regular icon, it replaced one regular icon with another.

Most likely, the AUMID of the window is getting changed at some point shortly after it has been painted. I don't have time to dive deeper at the moment, but I think that attaching a debugger and setting some breakpoints in the AUMID functions in https://searchfox.org/mozilla-central/source/widget/windows/WinTaskbar.cpp would make it pretty clear what's happening.

The fact that zip builds aren't typically launched with a shortcut with an AUMID associated with it may be playing a factor here.

(In reply to bhearsum@mozilla.com (:bhearsum) from comment #1)

I'm going to take at least a quick look at this regardless, but is this reproducible by starting with a newer version? Eg: does it happen if you go from 105.0b1 -> 105.0b2?

No, it doesn't happen in that case. I updated today a Beta build, from 105.0b1 to 105.0b3, without encountering the issue.

It sounds like this is no longer reproducible (I'm guessing it had something to do with older code that has since been fixed). Please re-open if it's still an issue.