This is a helper addon for the Tree Style Tab 3.4.0 and later, and depends on the Tab Extra Contents API introduced at TST 3.4.0.Due to technical reasons, it cannot be together to bookmark tabs and to detach tabs to a new window, when they are dragged from TST's sidebar. Detachable dragged tabs won't be bookmarked even if you drop them to the bookmarks toolbar, and bookmarkable tabs won't be detached even if you drop them outside the window. So TST provides ability to switch these actions by regular drag or dragging with the Shift key. This is annoying when you need to use only a mouse or something pointing device.
This helper addon provides small draggable handles on each tab, and it allows you to start dragging for each purpose: bookmarking or detaching.
This was a built-in feature of TST 2.6.0-3.3.6 but separated to a helper addon.
