test: add a bit more testing for vim.on_key() (#28095)

Also:
- Don't use NUMBUFLEN as buffer length as its unrelated.
- Restore accidentally removed comment from last commit.

src/nvim/getchar.c

@@ -1122,6 +1122,8 @@ static void gotchars(const uint8_t *chars, size_t len)
       pending--;
     }
 
+    // When receiving a special key sequence, store it until we have all
+    // the bytes and we can decide what to do with it.
     if ((pending == 0 || in_mbyte) && c == K_SPECIAL) {
       pending += 2;
       if (!in_mbyte) {

src/nvim/highlight_group.c

@@ -1914,7 +1914,7 @@ static void set_hl_attr(int idx)
   at_en.cterm_bg_color = (int16_t)sgp->sg_cterm_bg;
   at_en.rgb_ae_attr = (int16_t)sgp->sg_gui;
   // FIXME(tarruda): The "unset value" for rgb is -1, but since hlgroup is
-  // initialized with 0(by garray functions), check for sg_rgb_{f,b}g_name
+  // initialized with 0 (by garray functions), check for sg_rgb_{f,b}g_name
   // before setting attr_entry->{f,g}g_color to a other than -1
   at_en.rgb_fg_color = sgp->sg_rgb_fg_idx != kColorIdxNone ? sgp->sg_rgb_fg : -1;
   at_en.rgb_bg_color = sgp->sg_rgb_bg_idx != kColorIdxNone ? sgp->sg_rgb_bg : -1;

src/nvim/lua/executor.c

@@ -2066,7 +2066,7 @@ char *nlua_register_table_as_callable(const typval_T *const arg)
 
 void nlua_execute_on_key(int c)
 {
-  char buf[NUMBUFLEN];
+  char buf[MB_MAXBYTES * 3 + 4];
   size_t buf_len = special_to_buf(c, mod_mask, false, buf);
 
   lua_State *const lstate = global_lstate;

src/nvim/os/input.c

@@ -255,9 +255,9 @@ size_t input_enqueue(String keys)
   while (rbuffer_space(input_buffer) >= 19 && ptr < end) {
     // A "<x>" form occupies at least 1 characters, and produces up
     // to 19 characters (1 + 5 * 3 for the char and 3 for a modifier).
-    // In the case of K_SPECIAL(0x80), 3 bytes are escaped and needed,
+    // In the case of K_SPECIAL (0x80), 3 bytes are escaped and needed,
     // but since the keys are UTF-8, so the first byte cannot be
-    // K_SPECIAL(0x80).
+    // K_SPECIAL (0x80).
     uint8_t buf[19] = { 0 };
     // Do not simplify the keys here. Simplification will be done later.
     unsigned new_size

test/functional/lua/vim_spec.lua

@@ -3007,7 +3007,7 @@ describe('lua stdlib', function()
   end)
 
   describe('vim.on_key', function()
-    it('tracks keystrokes', function()
+    it('tracks Unicode input', function()
       insert([[hello world ]])
 
       exec_lua [[
@@ -3022,10 +3022,24 @@ describe('lua stdlib', function()
         end)
       ]]
 
-      insert([[next 🤦 lines å ]])
+      insert([[next 🤦 lines å …]])
 
       -- It has escape in the keys pressed
-      eq('inext 🤦 lines å <ESC>', exec_lua [[return table.concat(keys, '')]])
+      eq('inext 🤦 lines å …<ESC>', exec_lua [[return table.concat(keys, '')]])
+    end)
+
+    it('tracks input with modifiers', function()
+      exec_lua [[
+        keys = {}
+
+        vim.on_key(function(buf)
+          table.insert(keys, vim.fn.keytrans(buf))
+        end)
+      ]]
+
+      feed([[i<C-V><C-;><C-V><C-…><Esc>]])
+
+      eq('i<C-V><C-;><C-V><C-…><Esc>', exec_lua [[return table.concat(keys, '')]])
     end)
 
     it('allows removing on_key listeners', function()