Active tab selection not updating properly when the add-on opens with only one tab

When the add-on's sidebar opens with only one tab in the browser, the tab will properly mark as active. However when another tab is opened or activated the original tab will not get the `.active` class removed from it unless it is selected again. Therefore there is some problem either with the way the original tab is activated (using the `vtt clear, vtt init, vtt activate` loop-cycle), the tab's deactivation or the new tab's activation.

The same problem does not occur when two or more tabs are open in the browser before the sidebar is opened.

**P.S.:**First appeared on 582531e, which is when the code for tab activation/deactivation events first appeared.

Start by tracing the tab activation and deactivation event handlers introduced in commit 582531e, focusing on the vtt clear, vtt init, vtt activate loop-cycle. Reproduce the issue by opening the sidebar with one browser tab, then opening or activating another tab. Done means the original tab loses its .active class when the new tab becomes active.

Written by the indexing model from the issue text.