[Reverse tabs order] Tabs are wrongly positioned on top of the list when dragged and released in a specific area

[SoftVision-CiprianMuresan]

[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.
• "Reverse Tab Order" option is enabled from about:addons->Extensions->Tab Center.
• Have multiple tabs opened.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. Open a page in another window and drag the newly opened tab in the first window's Sidebar.
3. Release the mouse button on the bottom-half part of a tab.

[Expected result]:

• The tab is positioned above the hovered tab.

[Actual result]:

• The tab is positioned in the top most position of the sidebar.

[Notes]:

• The issue is also reproducible on the Test Pilot version of Tab Center.
• The issue is also reproducible when you duplicate tabs with Ctrl+Click.
• The issue is not reproducible with "reverse Tab Order" option disabled.
• Here is a screen recording of the issue:

[ericawright]

With the current version in master this has changed a little bit - the tabs now open roughly one slot below where they are supposed to open.