fix FLASK_DEBUG env var ignored with click 8.4.0 (#6025)

In click 8.4.0, `get_parameter_source` returns `None` during eager
callbacks instead of `ParameterSource.DEFAULT`. The guard in
`_set_debug` only checked for `source is not None`, so it fell through
and overwrote the user's `FLASK_DEBUG=1` env var with `"0"`.

Treat `source is None` the same as `DEFAULT` — return early without
overwriting the env var.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/flask/cli.py

@@ -470,7 +470,7 @@ def _set_debug(ctx: click.Context, param: click.Option, value: bool) -> bool | N
     # that, let debug be set by env in that case.
     source = ctx.get_parameter_source(param.name)
 
-    if source is not None and source in (
+    if source is None or source in (
         ParameterSource.DEFAULT,
         ParameterSource.DEFAULT_MAP,
     ):

tests/test_cli.py

@@ -382,6 +382,30 @@ def test_flaskgroup_debug(runner, set_debug_flag):
     assert result.output == f"{not set_debug_flag}\n"
 
 
+def test_flaskgroup_debug_env(monkeypatch, runner):
+    """FLASK_DEBUG=1 env var should not be overwritten when --debug is not
+    explicitly passed. Regression test for click 8.4.0 where
+    get_parameter_source returns None during eager callbacks.
+    """
+    monkeypatch.setenv("FLASK_DEBUG", "1")
+
+    def create_app():
+        app = Flask("flaskgroup")
+        return app
+
+    @click.group(cls=FlaskGroup, create_app=create_app)
+    def cli(**params):
+        pass
+
+    @cli.command()
+    def test():
+        click.echo(os.environ.get("FLASK_DEBUG"))
+
+    result = runner.invoke(cli, ["test"])
+    assert result.exit_code == 0
+    assert result.output.strip() == "1"
+
+
 def test_flaskgroup_nested(app, runner):
     cli = click.Group("cli")
     flask_group = FlaskGroup(name="flask", create_app=lambda: app)