term: insert-mode: handle combining characters correctly

When the client application emits combining characters, for example
multi-codepoint emojis, in insert-mode, we ended up pushing partial
graphemes to the right, for each codepoint, resulting in too many
cells (and with the wrong content) being inserted.

The fix is fairly simple; don't "insert" when appending characters to
an existing grapheme cluster.

This isn't something we can detect easily in print_insert() (it would
require us to do grapheme clustering again). Fortunately, we do have
the required information in action_utf8_print(). So, pass this
information as a boolean to term_print().

Closes #1947

CHANGELOG.md

@@ -105,9 +105,13 @@
   ([#1929][1929]).
 * Foot not closing file descriptors for unrecognized or `no_keymap`
   keymaps.
+* Combining characters (including emojis consisting of multiple
+  codepoints) not being handled correctly when _insert mode_ is
+  enabled ([#1947][1947]).
 
 [1918]: https://codeberg.org/dnkl/foot/issues/1918
 [1929]: https://codeberg.org/dnkl/foot/issues/1929
+[1947]: https://codeberg.org/dnkl/foot/issues/1947
 
 
 ### Security

csi.c

@@ -793,7 +793,7 @@ csi_dispatch(struct terminal *term, uint8_t final)
                 const int width = c32width(term->vt.last_printed);
                 if (width > 0) {
                     for (int i = 0; i < count; i++)
-                        term_print(term, term->vt.last_printed, width);
+                        term_print(term, term->vt.last_printed, width, false);
                 }
             }
             break;

terminal.c

@@ -3896,7 +3896,7 @@ term_fill(struct terminal *term, int r, int c, uint8_t data, size_t count,
 }
 
 void
-term_print(struct terminal *term, char32_t wc, int width)
+term_print(struct terminal *term, char32_t wc, int width, bool insert_mode_disable)
 {
     xassert(width > 0);
 
@@ -3918,7 +3918,8 @@ term_print(struct terminal *term, char32_t wc, int width)
     }
 
     print_linewrap(term);
-    print_insert(term, width);
+    if (!insert_mode_disable)
+        print_insert(term, width);
 
     int col = grid->cursor.point.col;
 
@@ -3990,7 +3991,7 @@ term_print(struct terminal *term, char32_t wc, int width)
 static void
 ascii_printer_generic(struct terminal *term, char32_t wc)
 {
-    term_print(term, wc, 1);
+    term_print(term, wc, 1, false);
 }
 
 static void

terminal.h

@@ -894,7 +894,8 @@ void term_cursor_up(struct terminal *term, int count);
 void term_cursor_down(struct terminal *term, int count);
 void term_cursor_blink_update(struct terminal *term);
 
-void term_print(struct terminal *term, char32_t wc, int width);
+void term_print(struct terminal *term, char32_t wc, int width,
+                bool insert_mode_disable);
 void term_fill(struct terminal *term, int row, int col, uint8_t c, size_t count,
                bool use_sgr_attrs);
 

vt.c

@@ -703,6 +703,7 @@ static void
 action_utf8_print(struct terminal *term, char32_t wc)
 {
     int width = c32width(wc);
+    bool insert_mode_disable = false;
     const bool grapheme_clustering = term->grapheme_shaping;
 
 #if !defined(FOOT_GRAPHEME_CLUSTERING)
@@ -757,6 +758,7 @@ action_utf8_print(struct terminal *term, char32_t wc)
         if (base_width > 0) {
             term->grid->cursor.point.col = col;
             term->grid->cursor.lcf = false;
+            insert_mode_disable = true;
 
             if (composed == NULL) {
                 bool base_from_primary;
@@ -954,7 +956,7 @@ action_utf8_print(struct terminal *term, char32_t wc)
 
 out:
     if (width > 0)
-        term_print(term, wc, width);
+        term_print(term, wc, width, insert_mode_disable);
 }
 
 static void