sl0ppty: measure chrome in columns, not characters

Good catch: a two-column bell mark shifted the pane title row. Everything in
app.c that positions chrome -- title budgets, button placement, the hit rects
that have to agree with them -- came from a `cells()` that counted codepoints,
and a glyph drawn in two columns was booked as one, so every glyph after it
landed a column early.

Widths now come from lib-vt's own table, via ghostty_unicode_grapheme_width.
That is deliberately the same table the terminal uses for pane content, so
chrome and content cannot disagree about how wide something is, and it is
cluster-aware: an emoji with a variation selector is two codepoints and one
glyph. A wide cell claims its right half as a tail (width 0), which the
renderer already knew how to skip.

HANDOVER said this was a known trap and to dump the actual cells before
touching arithmetic, which is exactly how this went: the first assertion I
wrote passed against a deliberately broken width function, because it measured
the frame's right corner -- drawn at an absolute position, so it does not move
-- rather than the columns the mark claims. The test now reads the mark's own
colour runs and demands 1 column for a narrow mark and 2 for a wide one, and it
fails when the width function is stubbed back to 1.

Two things fell out on the way:

ghostty_unicode_grapheme_width measures only the *first* cluster and returns
the codepoints it consumed. My first loop ignored that and asked "did the width
change when I added the next codepoint?", which concludes that an entire ASCII
string is a single cluster -- the cheatsheet modal collapsed to ten columns
wide with the text clipped. Use the return value.

A cluster can also be longer than a cell holds: a ZWJ family emoji is 18 bytes
against 16. Both the cell writer and the config's mark fields (now 32 bytes)
clamp on a codepoint boundary, because snprintf cuts at a byte and half a
codepoint is invalid UTF-8 -- which showed up as a decoder error in a test far
from the cause, not as a short mark.

1 files changed, 6 insertions, 3 deletions

diff --git a/src/config.h b/src/config.h
index 8c01fdb..36e9915 100644
--- a/src/config.h
+++ b/src/config.h
@@ -110,16 +110,19 @@ typedef struct {
    * reads as even: a glyph that floats in a mostly-empty cell donates its own
    * 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];
+  /* 32 bytes because a mark is a grapheme cluster, not a character: a
+   family emoji is eighteen bytes on its own. Truncating one mid-
+   codepoint used to put invalid UTF-8 on the wire. */
+  char zoom_mark[32], zoom_on_mark[32], close_mark[32], min_mark[32];
   /* 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];
+  char newtab_mark[32];
   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
    * terminals while chrome here is booked as one — so the default is narrow
    * and anyone who knows their terminal can say otherwise. */
-  char bell_mark[16];
+  char bell_mark[32];
   /* Which dead panes stay, showing what they printed and offering to run
    * again, and which just go.
    *