Opened new tab is not entirely visible on scrollable normal tabs list

[SoftVision-PaulOiegas]

[Affected versions]:

• Firefox 50.1.0 and above
• Tab Center 1.29.0

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• The "xpinstall.signatures.required" is set to "false" and latest Tab Center add-on is installed on a new clean profile.
• Default tabs list ordering is kept.
• Have multiple tabs opened so that the Sidebar is scroll-able.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. Scroll to the bottom of the tabs list.
3. Click on "New Tab" button or use "Ctrl + T" keyboard shortcut to open a new tab.
4. Observe how the tab is displayed in list.

[Expected result]:

• The list is scrolled down when the new tab appears to make it fully visible.

[Actual result]:

• The list is only partially scrolled down, leaving the new tab a bit hidden.

[Notes]:

• This issue is not reproducible with "reverse Tab Order" option set.
• Here is a screen recording of the issue: https://goo.gl/RZFzY4.