sidebar: align the numbers by default

The knob landed defaulting to "prefix" on the grounds that it is what the
strip has always drawn -- but the strip is not what this is about. A sidebar
row is wider than the name on it, and `1:api` spends the first two or three
cells of every row (three or four once you have ten tabs) on a number nobody
reads until they reach for C-a 2, pushing the names in by a distance that
depends on how many tabs are open. Right-aligned, the names start where the
statuses under them start and the numbers make a column of their own.

`tab_bar_index "prefix"` is still there and unchanged, and the top strip is
unaffected either way -- it ignores the setting, having no far end to align
against.

The tests that used `1:` as the anchor for "where a tab's name starts" now
anchor on the name, which is what they were about; two new ones cover the
default and the prefix form explicitly, so neither mode is tested only by
being the default.

5 files changed, 74 insertions, 38 deletions

diff --git a/config/config.kdl b/config/config.kdl
index 47473f0..acb4366 100644
--- a/config/config.kdl
+++ b/config/config.kdl
@@ -283,13 +283,18 @@ bell_mark "[!]"
 // The one-row bar with tabs, the new-tab button and the pane count.
 status_bar true
 
-// Where a tab's number sits. "prefix" writes it against the name (`1:api`);
-// "right" puts it at the far end of the row, so every name starts in the same
-// column and the numbers line up in one of their own -- which a sidebar has
-// room for and the top strip has not, a tab there being exactly as wide as
-// its label. Sidebars only for that reason, and a row too narrow to spare the
-// columns keeps the prefix: the name is the part you came for.
-tab_bar_index "prefix"
+// Where a tab's number sits in a *sidebar*. The default, "right", puts it at
+// the far end of the row, so every name starts in the same column and the
+// numbers line up in one of their own -- a list with a shape rather than a
+// column of digits in front of the words you are reading. "prefix" is the
+// older form, `1:api`, the number against the name.
+//
+// Sidebars only, and not by omission: a tab in the top strip is exactly as
+// wide as its label, so there is no far end to align against, and the setting
+// is ignored there rather than half-honoured. A sidebar row too narrow to
+// spare the columns also keeps the prefix -- the name is the part you came
+// for.
+tab_bar_index "right"
 
 // The line along the bottom: which session and tab you are in, what the
 // focused pane calls itself, and whether it is in a state worth knowing about
diff --git a/docs/panes.md b/docs/panes.md
index 3a28046..7f11fcd 100644
--- a/docs/panes.md
+++ b/docs/panes.md
@@ -55,7 +55,7 @@ The strip of tabs sits along the top by default. It does not have to:
 ```kdl
 tab_bar_side "left"   // or "right"; "top" is the default
 tab_bar_width 18      // columns the sidebar takes
-tab_bar_index "right" // the tab number at the far end, not against the name
+tab_bar_index "right" // the tab number at the far end; "prefix" is 1:name
 tab_bar_pad 0         // blank rows above the first tab
 tab_gap 0             // ...and between one tab and the next
 tab_bar_chrome true   // frame it like a pane
@@ -127,9 +127,9 @@ yours wins; the band is unset by default, which leaves a translucent
 terminal's own background alone. The top strip has one row and no room for any of this, which is half of
 why the sidebar exists.
 
-**The tab number can sit at the far end of the row.** `tab_bar_index "right"`
-draws `api` on the left and `3` hard against the right edge, instead of the
-default `3:api`:
+**A sidebar's tab numbers sit at the far end of the row**, `api` on the left
+and `3` hard against the right edge, rather than as the `3:api` the top strip
+writes. `tab_bar_index "prefix"` puts them back:
 
 ```
 │ tests        1 │        │ 1:tests        │
@@ -137,9 +137,9 @@ default `3:api`:
 │ api          3 │        │ 3:api          │
 ```
 
-Every name then starts in the same column and the numbers line up in one of
-their own, which is worth a column of a sidebar and is not available to the top
-strip at all — a tab there is exactly as wide as its label, so it has no far end
+Every name starts in the same column and the numbers line up in one of their
+own, which is worth a column of a sidebar and is not available to the top strip
+at all — a tab there is exactly as wide as its label, so it has no far end
 to align against, and the setting is ignored. A row too narrow to spare the
 columns goes back to the prefix, because the name is the part you came for, and
 a name long enough to reach the number always stops one cell short of it:
diff --git a/docs/slosh.5 b/docs/slosh.5
index ff8b386..3a467e5 100644
--- a/docs/slosh.5
+++ b/docs/slosh.5
@@ -136,9 +136,9 @@ keeps the full width. A terminal too narrow to give up the columns falls
 back to \fBtop\fR until it grows.
 .TP
 .BI tab_bar_index\  where
-Where a tab's number sits: \fBprefix\fR (the default) writes it against the
-name, \fB1:api\fR, and \fBright\fR puts it at the far end of the row so the
-names all start in one column and the numbers line up in another.
+Where a tab's number sits: \fBright\fR (the default) puts it at the far end
+of the row, so the names all start in one column and the numbers line up in
+another, and \fBprefix\fR writes it against the name, \fB1:api\fR.
 Sidebars only \(em a tab in the top strip is as wide as its label, so there
 is no far end to align against \(em and a row too narrow to spare the columns
 keeps the prefix, since the name is the part you came for.
diff --git a/src/config.c b/src/config.c
index 1297856..a1d8880 100644
--- a/src/config.c
+++ b/src/config.c
@@ -951,7 +951,7 @@ void config_defaults(config_t *c) {
   c->modal_scrim = 120;
   c->status_bar = true;
   c->tab_bar_side = TAB_BAR_TOP;
-  c->tab_bar_index = TAB_INDEX_PREFIX;
+  c->tab_bar_index = TAB_INDEX_RIGHT;
   /* Wide enough for " 12:a-real-name " plus a bell; narrow enough that a
    * 100-column terminal keeps a working layout beside it. */
   c->tab_bar_width = 18;
@@ -1711,8 +1711,8 @@ char *config_render(const config_t *c) {
          : c->tab_bar_side == TAB_BAR_RIGHT ? "right"
                                             : "top");
   cb_add(&b,
-         "tab_bar_index \"%s\"  // 1:name, or the number at the sidebar's far "
-         "edge\n",
+         "tab_bar_index \"%s\"  // the sidebar's number at its far edge, or "
+         "1:name\n",
          c->tab_bar_index == TAB_INDEX_RIGHT ? "right" : "prefix");
   cb_add(&b, "tab_bar_width %u      // columns a sidebar takes\n",
          c->tab_bar_width);
diff --git a/tests/test_tab_bar.py b/tests/test_tab_bar.py
index fca4742..b2220b2 100644
--- a/tests/test_tab_bar.py
+++ b/tests/test_tab_bar.py
@@ -158,14 +158,17 @@ def _status(s, text):
     s.settle(30)
 
 
+# The sidebar's default. LEFT would do as well -- the point of these is the
+# alignment, so they say which mode they mean rather than leaning on it.
 RIGHT_IDX = _cfg(f'tab_bar_side "left"\ntab_bar_width {W}\ntab_bar_index "right"\n')
+PREFIX_IDX = _cfg(f'tab_bar_side "left"\ntab_bar_width {W}\ntab_bar_index "prefix"\n')
 
 
-def test_the_index_can_sit_at_the_far_end():
-    """`tab_bar_index "right"`: names all start in one column, numbers line up
-    in another. A sidebar row is wider than the name on it, and the prefix
-    spends the start of every row on a number nobody reads until they want to
-    press it."""
+def test_the_index_sits_at_the_far_end():
+    """The sidebar's default: names all start in one column, numbers line up in
+    another. A sidebar row is wider than the name on it, and a prefix spends
+    the start of every row on a number nobody reads until they want to press
+    it."""
     with Session(SH, cols=90, rows=20, config=RIGHT_IDX) as s:
         s.settle(30)
         s.api("new-tab", name="api")
@@ -246,15 +249,29 @@ def test_the_whole_row_is_still_one_door():
         )
 
 
+def test_the_prefix_is_still_there_for_anyone_who_wants_it():
+    """`1:name`, which is what the top strip draws and what the sidebar drew
+    before the alignment existed."""
+    with Session(SH, cols=90, rows=20, config=PREFIX_IDX) as s:
+        s.settle(30)
+        row = _row(s.snapshot(), Y0)
+        check("the number is in front of the name", row.strip() == "1:tests", repr(row))
+
+
+def test_the_sidebar_aligns_by_default():
+    with Session(SH, cols=90, rows=20, config=LEFT) as s:
+        s.settle(30)
+        row = _row(s.snapshot(), Y0)
+        check("no config asked for it", row.rstrip()[-1] == "1", repr(row))
+        check("...and no prefix came with it", "1:" not in row, repr(row))
+
+
 def test_a_bad_index_mode_is_a_line_and_no_more():
     bad = _cfg(f'tab_bar_side "left"\ntab_bar_width {W}\ntab_bar_index "middle"\n')
     with Session(SH, cols=90, rows=20, config=bad) as s:
         s.settle(30)
-        check(
-            "the session runs, with the default",
-            "1:tests" in s.snapshot().screen(),
-            _row(s.snapshot(), Y0),
-        )
+        row = _row(s.snapshot(), Y0)
+        check("the session runs, with the default", row.rstrip()[-1] == "1", repr(row))
 
 
 def test_every_tab_sits_on_a_plate():
@@ -367,8 +384,8 @@ def test_status_rows_are_told_apart_by_the_slant_not_by_an_indent():
         # starts in. Three columns of indent is a fifth of this sidebar.
         check(
             "a status is not indented under its label",
-            status.index("building") == label.index("1:"),
-            f"{status.index('building')} vs {label.index('1:')}",
+            status.index("building") == label.index("tests"),
+            f"{status.index('building')} vs {label.index('tests')}",
         )
         st = snap.style_at(CX + 1, Y0 + 1)
         check("it is italic instead", "italic" in st["attrs"], str(st))
@@ -729,7 +746,11 @@ def test_an_unnamed_tab_borrows_its_directory():
     with Session(cd, cols=90, rows=20, config=LEFT) as s:
         s.settle(60)
         row = _row(s.snapshot(), Y0)
-        check("the label is the focused pane's dir", "1:tmp" in row, repr(row))
+        check(
+            "the label is the focused pane's dir",
+            row.split() == ["tmp", "1"],
+            repr(row),
+        )
 
 
 def test_a_real_name_beats_the_derived_one():
@@ -739,17 +760,25 @@ def test_a_real_name_beats_the_derived_one():
     with Session(SH, cols=90, rows=20, config=LEFT, layout=lay.name) as s:
         s.settle(60)
         row = _row(s.snapshot(), Y0)
-        check("a declared name is not second-guessed", "1:api" in row, repr(row))
+        check(
+            "a declared name is not second-guessed",
+            row.split() == ["api", "1"],
+            repr(row),
+        )
 
 
 def test_the_borrowed_name_follows_a_cd():
     loop = ["/bin/sh", "-c", 'cd /tmp && while IFS= read -r l; do eval "$l"; done']
     with Session(loop, cols=90, rows=20, config=LEFT) as s:
-        s.until(lambda snap: "1:tmp" in snap.line(Y0))
+        s.until(lambda snap: "tmp" in snap.line(Y0))
         s.send(r"cd /home\n")
-        s.until(lambda snap: "1:home" in snap.line(Y0))
+        s.until(lambda snap: "home" in snap.line(Y0))
         row = _row(s.snapshot(), Y0)
-        check("the kernel's answer moves the label", "1:home" in row, repr(row))
+        check(
+            "the kernel's answer moves the label",
+            row.split() == ["home", "1"],
+            repr(row),
+        )
 
 
 def test_the_sidebar_is_framed_like_a_pane():
@@ -957,7 +986,9 @@ if __name__ == "__main__":
     test_sidebar_rows_click_like_the_strip()
     test_right_sidebar_takes_the_other_edge()
     test_pad_pushes_the_list_down()
-    test_the_index_can_sit_at_the_far_end()
+    test_the_index_sits_at_the_far_end()
+    test_the_prefix_is_still_there_for_anyone_who_wants_it()
+    test_the_sidebar_aligns_by_default()
     test_a_long_name_stops_short_of_the_number()
     test_a_narrow_row_keeps_the_prefix()
     test_the_top_strip_ignores_it()