Zero-based index of the layer tab the keyboard cursor sits on.
Distinct from the layer the dialog is describing (data.layers' active
entry): the tab strip activates manually, so arrowing along it moves this
cursor and nothing else until Space confirms the choice. Without the two
being separate, every arrow keypress would switch the reader's layer in
the chart behind the dialog.
-1 when there is nothing to point at — no description open, or a single-layer subplot, which gets no tab strip.
State interface for the chart description modal.