fix(f_wait): flush UI before blocking (#25962)

src/nvim/eval/funcs.c

@@ -2911,6 +2911,9 @@ static void f_wait(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
   bool error = false;
   const int called_emsg_before = called_emsg;
 
+  // Flush screen updates before blocking.
+  ui_flush();
+
   LOOP_PROCESS_EVENTS_UNTIL(&main_loop, main_loop.events, timeout,
                             eval_expr_typval(&expr, false, &argv, 0, &exprval) != OK
                             || tv_get_number_chk(&exprval, &error)

src/nvim/getchar.c

@@ -1723,7 +1723,7 @@ static void getchar_common(typval_T *argvars, typval_T *rettv)
       // getchar(): blocking wait.
       // TODO(bfredl): deduplicate shared logic with state_enter ?
       if (!char_avail()) {
-        // flush output before waiting
+        // Flush screen updates before blocking.
         ui_flush();
         (void)os_inchar(NULL, 0, -1, typebuf.tb_change_cnt, main_loop.events);
         if (!multiqueue_empty(main_loop.events)) {

src/nvim/lua/executor.c

@@ -463,7 +463,7 @@ static int nlua_wait(lua_State *lstate)
   int pcall_status = 0;
   bool callback_result = false;
 
-  // Flush UI before blocking
+  // Flush screen updates before blocking.
   ui_flush();
 
   LOOP_PROCESS_EVENTS_UNTIL(&main_loop,

src/nvim/state.c

@@ -70,7 +70,7 @@ getkey:
         update_screen();
         setcursor();  // put cursor back where it belongs
       }
-      // Flush screen updates before blocking
+      // Flush screen updates before blocking.
       ui_flush();
       // Call `os_inchar` directly to block for events or user input without
       // consuming anything from `input_buffer`(os/input.c) or calling the