to reproduce: try to drag a tab to the right of the rightmost app-tab, with the intend to make it the new rightmost app-tab. it does not work here, the arrow indicating where the tab will go will be to the laft of the leftmost normal tab. when the tab bar is not overflown, this is somewhat reasonable, but its still not perfect as you can't make a tab the rightmost app-tab by dragging it. when the tab bar is overflown, this is weird, as the overflow widget marks a clear border between app-tabs and normal tabs and a user would expect that indicator-arrow to be to the right of the rightmost app-tab when trying to drag a tab to the left of the overflow widget.
Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:13.0) Gecko/20120301 Firefox/13.0a1 Mozilla/5.0 (Windows NT 6.1; WOW64; rv:11.0) Gecko/20100101 Firefox/11.0 This issue reproduces on the above builds.
it seems like it is impossible to make a tab an app-tab per dragndrop in australis at all. if this is intended, this bug can be closed.
actually this is still relevant for tabs dragged from other windows