fix(input): discard following keys when discarding <Cmd>/K_LUA (#36498)

Technically the current behavior does match documentation. However, the
keys following <Cmd>/K_LUA aren't normally received by vim.on_key()
callbacks either, so it does makes sense to discard them along with the
preceding key.

One may also argue that vim.on_key() callbacks should instead receive
the following keys together with the <Cmd>/K_LUA, but doing that may
cause some performance problems, and even in that case the keys should
still be discarded together.

src/nvim/edit.c

@@ -986,7 +986,7 @@ static int insert_handle_key(InsertState *s)
     goto check_pum;
 
   case K_LUA:
-    map_execute_lua(false);
+    map_execute_lua(false, false);
 
 check_pum:
     // nvim_select_popupmenu_item() can be called from the handling of

src/nvim/ex_getln.c

@@ -1296,7 +1296,7 @@ static int command_line_execute(VimState *state, int key)
     } else if (s->c == K_COMMAND) {
       do_cmdline(NULL, getcmdkeycmd, NULL, DOCMD_NOWAIT);
     } else {
-      map_execute_lua(false);
+      map_execute_lua(false, false);
     }
     // If the window changed incremental search state is not valid.
     if (s->is_state.winid != curwin->handle) {

src/nvim/getchar.c

@@ -1798,6 +1798,16 @@ int vgetc(void)
   // Execute Lua on_key callbacks.
   kvi_push(on_key_buf, NUL);
   if (nlua_execute_on_key(c, on_key_buf.items)) {
+    // Keys following K_COMMAND/K_LUA/K_PASTE_START aren't normally received by
+    // vim.on_key() callbacks, so discard them along with the current key.
+    if (c == K_COMMAND) {
+      xfree(getcmdkeycmd(NUL, NULL, 0, false));
+    } else if (c == K_LUA) {
+      map_execute_lua(false, true);
+    } else if (c == K_PASTE_START) {
+      paste_repeat(0);
+    }
+    // Discard the current key.
     c = K_IGNORE;
   }
   kvi_destroy(on_key_buf);
@@ -3213,9 +3223,10 @@ char *getcmdkeycmd(int promptc, void *cookie, int indent, bool do_concat)
 /// Handle a Lua mapping: get its LuaRef from typeahead and execute it.
 ///
 /// @param may_repeat  save the LuaRef for redoing with "." later
+/// @param discard     discard the keys instead of executing the LuaRef
 ///
 /// @return  false if getting the LuaRef was aborted, true otherwise
-bool map_execute_lua(bool may_repeat)
+bool map_execute_lua(bool may_repeat, bool discard)
 {
   garray_T line_ga;
   int c1 = -1;
@@ -3241,9 +3252,9 @@ bool map_execute_lua(bool may_repeat)
 
   no_mapping--;
 
-  if (aborted) {
+  if (aborted || discard) {
     ga_clear(&line_ga);
-    return false;
+    return !aborted;
   }
 
   LuaRef ref = (LuaRef)atoi(line_ga.ga_data);

src/nvim/normal.c

@@ -3191,7 +3191,7 @@ static void nv_colon(cmdarg_T *cap)
   }
 
   if (is_lua) {
-    cmd_result = map_execute_lua(true);
+    cmd_result = map_execute_lua(true, false);
   } else {
     // get a command line and execute it
     cmd_result = do_cmdline(NULL, is_cmdkey ? getcmdkeycmd : getexline, NULL,

src/nvim/terminal.c

@@ -986,7 +986,7 @@ static int terminal_execute(VimState *state, int key)
     break;
 
   case K_LUA:
-    map_execute_lua(false);
+    map_execute_lua(false, false);
     break;
 
   case Ctrl_N: