Dragging a tab showed no floating copy: the ghost cloned the tab's classes, and .tab { position: relative } loaded after .ghost { position: fixed }. The ghost is now positioned inline, and a tab dragged within its strip stays as an empty slot the other tabs move around; the insertion line for tabs from another strip is drawn inside the edge tabs so the scroller doesn't clip it. A terminal group is now a drop target like an editor pane: the middle joins the group, an edge splits off a new group (dropZoneAt moves to ui and is shared).
Closes#1052
Co-authored-by: fuqiuluo <61898844+fuqiuluo@users.noreply.github.com>