The split tabs are wrongly separated if you use "Open Link in a New Tab" option from the 1st split tab

[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.

[Steps to reproduce]:

1. Open Firefox browser with the profile from prerequisites and navigate to https://www.wikipedia.org/.
2. Click the "Tab Split" button from the toolbar and navigate to a website.
3. Click the Wikipedia tab and right click a hyperlinked element (eg. "English").
4. Select the "Open in a New Tab" option and observe the behavior.

[Expected result]:

• The new tab is positioned after the split tabs.

[Actual result]:

• The split tabs are separated and the new tab is opened between them.

[Notes]:

• After focusing a tab opened this way, the split tabs will reunite.
• This issue is not reproducible if the second tab is focused in the tab strip.
• This issue is not reproducible if you use the "Open in a New Tab" option from the 2nd split tab.
• Here is a screen recording with this issue:

[ianb]

This feels related to #72