fix(getchar): flush screen before doing a blocking wait

src/nvim/eval/funcs.c

@@ -2722,6 +2722,8 @@ 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
+        ui_flush();
         (void)os_inchar(NULL, 0, -1, 0, main_loop.events);
         if (!multiqueue_empty(main_loop.events)) {
           state_handle_k_event();

test/functional/ui/messages_spec.lua

@@ -1243,6 +1243,19 @@ vimComment     xxx match /\s"[^\-:.%#=*].*$/ms=s+1,lc=1  excludenl contains=@vim
                                                                   |
     ]])
   end)
+
+  it('echo messages are shown correctly when getchar() immediately follows', function()
+    feed([[:echo 'foo' | echo 'bar' | call getchar()<CR>]])
+    screen:expect([[
+                                                                  |
+      {1:~                                                           }|
+      {1:~                                                           }|
+      {1:~                                                           }|
+      {3:                                                            }|
+      foo                                                         |
+      bar^                                                         |
+    ]])
+  end)
 end)
 
 describe('ui/ext_messages', function()