The middle bar wrongly remains for a brief moment on tabs opened from split tabs

[Softvision-CristinaBadescu]

[Affected versions]:

• Nightly 60.0a1
• Tab Split 0.1.7

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a clean Firefox profile with the latest Tab Split v0.1.7 installed.
• Have 1 tab open.

[Steps to reproduce]:

1. Open Firefox browser with the profile from prerequisites.
2. Click the "Tab Split" button from the toolbar.
3. Right click a hyperlinked element and select the "Open Link in New Tab" context menu option.
4. Click the opened tab and observe the behavior.

[Expected result]:

• The page is displayed.

[Actual result]:

• The bar between the two split tabs is displayed for a brief moment.

[Notes]:

• This issue is also reproducible if you open the link by using the middle scroll mouse button.

• This issue is not reproducible if you open links like this from non split tabs.

• It seems that the bar appears based on its previous position between the two split tabs.

• This issue is easy to be noticed on pages with colored background.

• Here is a screenshot with this issue:
  

• Here is a screen recording with this issue: