sl0ppty: the new-tab button is a bare +, and yours to choose

It read "+tab" because of this, in the code, right above it:

    Not a bare "+": the frame already has one, for splitting a pane. Two
    verbs that look identical is a UI bug the fork shipped and noticed.

That was true when it was written and has not been since f2f3d67, which
removed the frame's "+" when the border became the button. The collision it
was avoiding no longer exists, and the word was left explaining itself to
nobody -- while every other affordance in this UI is a bare mark: _ # x on a
frame, and a hint under the pointer for anyone who wants the word.

So: "+", padded to three cells, because their own comment applies -- a
one-cell target is a thing you miss with a mouse.

Also configurable, as `newtab_mark`, which it should have been already: it
was the only mark here that was not (zoom_mark, zoom_on_mark, close_mark,
min_mark, bell_mark all are). Try "[+]" or "new" if you disagree; a session
picks it up on save.

Four tests were asserting the literal "+tab". Three of them only needed the
new string, but two were using it to *locate* the button -- bar.index("+tab")
-- which is exactly the mistake of asserting something incidental: what the
button says is now config, and a test that spells it out fails the day
somebody changes their mind. They look it up through the hit list now, which
is what they meant.

test_no_more_plus keeps the invariant that makes the bare mark safe, and now
says so: there is exactly one "+" in the UI, so it means one thing. If one
ever returns to a frame, that button needs its word back.

3 files changed, 27 insertions, 7 deletions

diff --git a/src/app.c b/src/app.c
index be707c7..785d028 100644
--- a/src/app.c
+++ b/src/app.c
@@ -2559,14 +2559,28 @@ static void draw_tab_strip(app_t *a, screen_t *s) {
     x = (uint16_t)(x + w);
   }
 
-  /* Not a bare "+": the frame already has one, for splitting a pane. Two
-   * verbs that look identical is a UI bug the fork shipped and noticed. */
-  if (x + 6 < right) {
-    uint16_t w = screen_text(s, x, y, " +tab ", TAB_IDLE, NO_COLOR, 0);
-    if (ptr_on(a, x, y, w, 1))
-      screen_text(s, x, y, " +tab ", TAB_HOVER, NO_COLOR, ATTR_BOLD);
-    hit_add(&s->hits, x, y, w, 1, "newtab");
+  /* A bare mark, spaced like the frame's own buttons rather than spelled out.
+   * It used to read "+tab", because a pane frame carried a "+" for splitting
+   * and two verbs that look identical is a UI bug the fork shipped. That "+"
+   * went when the border became the button, so the collision it was avoiding
+   * no longer exists and the word was left explaining itself to nobody.
+   *
+   * Padded to three cells for the same reason the frame's buttons are: a
+   * one-cell target is a thing you miss with a mouse. What it does is said by
+   * the hint under the pointer, which is where every other one-character
+   * affordance here says it. */
+  {
+    char btn[24];
+    snprintf(btn, sizeof btn, " %s ", CFG.newtab_mark);
+    uint16_t bw = (uint16_t)(cells(CFG.newtab_mark) + 2);
+    if (bw > 2 && x + bw <= right) {
+      uint16_t w = screen_text(s, x, y, btn, TAB_IDLE, NO_COLOR, 0);
+      if (ptr_on(a, x, y, w, 1))
+        screen_text(s, x, y, btn, TAB_HOVER, NO_COLOR, ATTR_BOLD);
+      hit_add(&s->hits, x, y, w, 1, "newtab");
+    }
   }
+
 }
 
 /* The space between two of a split's children, or false if they are flush.
diff --git a/src/config.c b/src/config.c
index 2b80a18..80d9936 100644
--- a/src/config.c
+++ b/src/config.c
@@ -177,6 +177,7 @@ void config_defaults(config_t *c) {
   snprintf(c->zoom_on_mark, sizeof c->zoom_on_mark, "*");
   snprintf(c->close_mark, sizeof c->close_mark, "x");
   snprintf(c->min_mark, sizeof c->min_mark, "_");
+  snprintf(c->newtab_mark, sizeof c->newtab_mark, "+");
   c->bell_indicator = true;
   snprintf(c->bell_mark, sizeof c->bell_mark, "\u2022");
   c->keep_dead = true;
@@ -400,6 +401,8 @@ bool config_load(config_t *c, const char *path, char *err, size_t errcap) {
   if (cm) snprintf(c->close_mark, sizeof c->close_mark, "%s", cm);
   const char *mm = kdl_arg(kdl_child(root, "min_mark"), 0, NULL);
   if (mm) snprintf(c->min_mark, sizeof c->min_mark, "%s", mm);
+  const char *nt = kdl_arg(kdl_child(root, "newtab_mark"), 0, NULL);
+  if (nt) snprintf(c->newtab_mark, sizeof c->newtab_mark, "%s", nt);
   c->keep_dead = kdl_arg_bool(kdl_child(root, "keep_dead"), 0, c->keep_dead);
   c->bell_indicator =
       kdl_arg_bool(kdl_child(root, "bell_indicator"), 0, c->bell_indicator);
diff --git a/src/config.h b/src/config.h
index f8b799c..4be4ae0 100644
--- a/src/config.h
+++ b/src/config.h
@@ -97,6 +97,9 @@ typedef struct {
    * whitespace to the gap beside it and looks further away than its
    * neighbours, which is what an en dash did here. */
   char zoom_mark[16], zoom_on_mark[16], close_mark[16], min_mark[16];
+  /* The new-tab button at the end of the strip. Drawn with a space each side,
+   * so a one-character mark is still a three-cell target. */
+  char newtab_mark[16];
   bool bell_indicator;
   /* What to draw. A short string rather than a fixed glyph: the obvious
    * choice is an emoji bell, and emoji are drawn two columns wide by many