term: append zero-width grapheme breaking characters to previous cell

When compiled with grapheme clustering support, zero-width characters
that also are grapheme breaks, were ignored (not stored in the
grid).

When utf8proc says the character is a grapheme break, we try to print
the character to the current cell. But this is only done when width >
0. As a result, zero width grapheme breaks were simply discarded.

This only happens when grapheme clustering is enabled; when disabled,
all zero width characters are appended.

Fix this by also requiring the width to be non-zero when if we should
append the character or not.

Closes #1960

CHANGELOG.md

@@ -112,11 +112,15 @@
   enabled ([#1947][1947]).
 * Reflow of the cursor (active + saved) when at the end of the line
   with a pending wrap (LCF set) ([#1954][1954]).
+* Zero-width characters that also are grapheme breaks (e.g. U+200B,
+  ZERO WIDTH SPACE) being ignored (discarded and never stored in the
+  grid) ([#1960][1960]).
 
 [1918]: https://codeberg.org/dnkl/foot/issues/1918
 [1929]: https://codeberg.org/dnkl/foot/issues/1929
 [1947]: https://codeberg.org/dnkl/foot/issues/1947
 [1954]: https://codeberg.org/dnkl/foot/issues/1954
+[1960]: https://codeberg.org/dnkl/foot/issues/1960
 
 
 ### Security

terminal.c

@@ -4153,7 +4153,7 @@ term_process_and_print_non_ascii(struct terminal *term, char32_t wc)
         if (grapheme_clustering) {
             /* Check if we're on a grapheme cluster break */
             if (utf8proc_grapheme_break_stateful(
-                    last, wc, &term->vt.grapheme_state))
+                    last, wc, &term->vt.grapheme_state) && width > 0)
             {
                 term_reset_grapheme_state(term);
                 goto out;