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/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;

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()