Problem:
- Current 'statuscolumn' click label caveat is restrictive.
- v:virtnum is not unique to a line if it has both above and
below virtual lines.
- 'statuscolumn' click handler may expect v:virt/lnum to be set.
Solution:
- Store per-row click definitions for the statuscolumn in a
(nested line/virt number) map.
- Implement strategy that gives each 'statuscolumn' row a unique
v:virtnum.
- Set v:virt/lnum when determining which line is clicked.