Handle help arg by itself the same as no args

When the 'flask' command is used with only the '--help' parameter, this
change will make sure to try and load the app before the help callback
is run. This was previously only being done when the 'flask' command was
used by itself. This meant when passing in '--help', any custom commands
were not getting shown in the help message. With this change, custom
commands will be included in the help message when running 'flask' on
the command line by itself or with the '--help' parameter.

CHANGES.rst

@@ -4,6 +4,8 @@ Version 3.1.1
 Unreleased
 
 -   Fix type hint for `cli_runner.invoke`. :issue:`5645`
+-   ``flask --help`` loads the app and plugins first to make sure all commands
+    are shown. :issue:5673`
 
 
 Version 3.1.0

src/flask/cli.py

@@ -684,7 +684,9 @@ class FlaskGroup(AppGroup):
         return super().make_context(info_name, args, parent=parent, **extra)
 
     def parse_args(self, ctx: click.Context, args: list[str]) -> list[str]:
-        if not args and self.no_args_is_help:
+        if (not args and self.no_args_is_help) or (
+            len(args) == 1 and args[0] in self.get_help_option_names(ctx)
+        ):
             # Attempt to load --env-file and --app early in case they
             # were given as env vars. Otherwise no_args_is_help will not
             # see commands from app.cli.