tabs: the strip can stand on its side (tab_bar_side left/right)

A sidebar lists one tab per row down the chosen edge, tab_bar_width
columns wide; the panes trade the strip's row for the sidebar's columns
in tab_area(), the one place that arithmetic lives. The per-tab cell
drawing is extracted from draw_tab_strip (label, rename editor, bell,
hover, drop states, hit) and shared, so every mouse verb -- click,
double-click rename, drag to a tab, drop on + -- works in both shapes
because the hits move with the cells.

The strip's indicators (pane count, prefix badge) take the sidebar's
bottom rows, reserved first under the same budgeting rule the strip
applies from the right. The status line below keeps the full width. A
terminal too narrow to give up the columns falls back to top, re-derived
per frame, so a resize moves the bar without anyone storing state.

1 files changed, 7 insertions, 0 deletions

diff --git a/docs/slosh.5 b/docs/slosh.5
index 8e282f3..0986934 100644
--- a/docs/slosh.5
+++ b/docs/slosh.5
@@ -127,6 +127,13 @@ a split is not offered in the first place.
 The tab strip along the top, the status line along the bottom, and the
 line's side margin.
 .TP
+.BR tab_bar_side\  \fIside\fR ,\ tab_bar_width\  \fIn\fR
+Where the tab strip lives: \fBtop\fR (the default), or a sidebar on the
+\fBleft\fR or \fBright\fR listing one tab per row, \fIn\fR columns wide.
+The strip's indicators move to the sidebar's bottom rows; the status line
+keeps the full width. A terminal too narrow to give up the columns falls
+back to \fBtop\fR until it grows.
+.TP
 .BI hints\  bool
 What the pointer is resting on, said in the status line.
 .TP