Multi-account-containers: "Multi-Account Containers" toolbar button is wrongly active in Private Window

[SoftVision-EmilPasca]

[Affected versions]:

• Multi-Account Containers 4.0.3
• Firefox 58.0a1

[Affected Platforms]:

• All Windows
• All Mac (No longer reproducible 58.0a1-20171102100041)
• All Linux

[Prerequisites]:

• Have a Firefox profile with the latest "Multi-Account Containers" add-on version (4.0.3) installed.

[Steps to reproduce]:

1. Open the browser with the profiles from prerequisites.
2. Open a new Private Window (Ctrl+Shift+P).
3. Click the "Multi-Account Containers" from the browser toolbar and observe.

[Expected result]:

• Nothing happens.

[Actual result]:

• The "Containers" panel is opened.

[Regression Window on Windows]:

• Last good revision: 71ea88d938e5d68c5843a3689b5ddc0fda38a063
• First bad revision: dda6098cc5161046af377ef69bae69099255b098
• Pushlog https://goo.gl/eAuBnK

[Regression Window on Mac]:

• First good revision: a0334f789772302ba5cfb6fd61290408842c7432
• Last bad revision: 0a50066b2de805de843762b9787b2b3d2d5bf93f
• Pushlog: https://goo.gl/NiAyM4
• It seems that the backed out from this pushlog made the issue no longer reproducible.

[Notes]:

• Attached a screen recording of the issue:
  

[Softvision-MariusComan]

An user can also encounter the following issue if he chooses to show previously hidden containers from a "Private Window":

[Notes]:

• This scenario will also break the "Show this container" option when clicked in a normal window.

[Prerequisites]:

• Have a Firefox profile with the latest "Multi-Account Containers" add-on version (4.0.3 custom built on 11.08.2017) installed.
• Have a container hidden.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. Open a new Private Window (Ctrl+Shift+P) and click the "Multi-Account Containers" button from the browser toolbar.
3. Click the "View container" button (">") placed in the right of the container from prerequisites.
4. Click the "Show this container" option and observe the behavior.

[Expected result]:

• The hidden container tabs are displayed.

[Actual result]:

• Nothing happens.

[Additional Notes]:

• Attached a screen recording of the issue:
  

[yatri1609]

@groovecoder Can I take this issue if it is free?

[groovecoder]

Yes, you can take it. Note: this fix here is not to make the extension work in Private Browsing mode - I believe that requires platform-level work.

The fix here is to disable the containers extension icon in private browsing windows.

[yatri1609]

@groovecoder I was thinking if we can show a message on clicking the button that this is not supported in Private browsing then it will be good. What do you think?

[romanrodriguez]

Hi @maxxcrawford & @groovecoder, I'd love to work on this issue for my Outreachy application. :)

[AlbionaHoti]

If @romanrodriguez is not available to work on this task, I would want to continue. I am an Outreachy applicant.

[maxxcrawford]

@romanrodriguez Please proceed. Assigned!

@AlbionaHoti You are next in line.

Thank you both for your interest!