The "Page Action" area is gradually moved to the left each time when a tab is opened in foreground

[Softvision-MariusComan]

[Affected Versions]:

• Unbranded Firefox Beta (59.0b14) Build ID:20180301022608

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a new Firefox profile.
• Have the following prefs. in the "about:config" page (If a preference does not exist, please create it):
  • “extensions.legacy.enabled” pref set to “true”;
  • “xpinstall.signatures.required” pref set to “false”;
  • “extensions.tracking_protection_messaging_study.variation_override” pref set to "fast".
• Have the latest "Tracking Protection Messaging Shield Study" add-on (1.0.3) installed.
• Have the "When you open a link in a new tab, switch to it immediately" option checked in the "about:preferences" page.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites and navigate to any website (eg. https://www.reddit.com/).
2. Open a link in a new tab and switch back to the first.
3. Repeat step 2 at least 6 times and observe the address bar each time.

[Expected result]:

• All elements are in their places.

[Actual result]:

• The "Page action", "Save to Pocket" and "Bookmark" buttons are gradually moved to the left part of the address bar.

[Notes]:

• The issue is reproducible only with pages where the "pageAction Badge" is displayed.
• The issue is also reproducible with the “extensions.tracking_protection_messaging_study.variation_override” pref set to "private".
• The "Page action", "Save to Pocket" and "Bookmark" buttons are correctly placed in a new window.
• The "Page action", "Save to Pocket" and "Bookmark" buttons are reset in all affected windows after a browser restart.
• Attached a screen recording of the issue:

[biancadanforth]

Wow that's really weird. I never noticed. Will check it out; thanks.

[SoftVision-EmilPasca]

This issue is no longer reproducible with the latest Tracking Protection Messaging .xpi installed (v.1.0.5), retested on Windows 10 x64, Mac OS 10.13 and Ubuntu 16.04 with Unbranded Beta 59.0b14.