fix(input): resolve isolated (non-ALT/META) mappings #13109

Problem:
Since 2f06413dfb36 #13042, "ESC+c" sequence is treated as "ESC c"
instead of "M-c" (ALT/META+c) when not mapped, aka "fallthrough"
behavior. But "isolated" (non-ALT/META) mappings to ESC and c were not
resolved. This behavior is especially confusing for the TUI.

Solution:
Resolve isolated ESC, c mappings when there is no M-c mapping.
Change ins_char_typebuf() to escape CSI, K_SPECIAL.

fixes #13086
fixes #15869

1 files changed, 15 insertions, 3 deletions

diff --git a/src/nvim/getchar.c b/src/nvim/getchar.c
index beb4ff4da6..16ad890360 100644
--- a/src/nvim/getchar.c
+++ b/src/nvim/getchar.c
@@ -1000,6 +1000,18 @@ void ins_char_typebuf(int c)
     buf[3] = NUL;
   } else {
     buf[utf_char2bytes(c, buf)] = NUL;
+    char_u *p = buf;
+    while (*p) {
+      if ((uint8_t)(*p) == CSI || (uint8_t)(*p) == K_SPECIAL) {
+        bool is_csi = (uint8_t)(*p) == CSI;
+        memmove(p + 3, p + 1, STRLEN(p + 1) + 1);
+        *p++ = K_SPECIAL;
+        *p++ = is_csi ? KS_EXTRA : KS_SPECIAL;
+        *p++ = is_csi ? KE_CSI : KE_FILLER;
+      } else {
+        p++;
+      }
+    }
   }
   (void)ins_typebuf(buf, KeyNoremap, 0, !KeyTyped, cmd_silent);
 }
@@ -1573,9 +1585,9 @@ int vgetc(void)
       if (!no_mapping && KeyTyped
           && (mod_mask == MOD_MASK_ALT || mod_mask == MOD_MASK_META)) {
         mod_mask = 0;
-        stuffcharReadbuff(c);
-        u_sync(false);
-        c = ESC;
+        ins_char_typebuf(c);
+        ins_char_typebuf(ESC);
+        continue;
       }
 
       break;