I've taken a more detailed look at the tabs. I like the clean look and
the functionality very much.

To make multi-selecting tabs perfect:
- I would like to be able to shift-click to multi-select;

- if I command-click on a already selected tab, I would like it to
 be removed from the set of selected tabs;

- I would like the horizontal line below the selected tabs to go away,
 to make clearer that I'm looking at multiple tabs;

- when I resize a column, I would like the relative size to stay
 when I add another one;

- I'm not sure making the order of the panes dependent on the order in which
 the tabs are selected is a good idea, especially when you allow reordering
 tabs by drag-and-drop; I think position based ordering is easier to understand;

- one way of showing what pane belongs to which tab, would be to draw
 a thin colored line from the tab to the pane; there is some vertical space
 you could use for that;

- another would be to put a (opaque) icon with an arrow (in a circle?) pointing
 at the center of the pane over the tab.

 

Stephan