Fix misdetection of project root with --stdin-filename

CHANGES.md

@@ -34,6 +34,8 @@
 <!-- Changes to how Black can be configured -->
 
 - Black now uses the presence of debug f-strings to detect target version. (#3215)
+- Fix misdetection of project root and verbose logging of sources in cases involving
+  `--stdin-filename` (#3216)
 
 ### Documentation
 

src/black/__init__.py

@@ -469,7 +469,13 @@ def main(  # noqa: C901
         out(main.get_usage(ctx) + "\n\nOne of 'SRC' or 'code' is required.")
         ctx.exit(1)
 
-    root, method = find_project_root(src) if code is None else (None, None)
+    if stdin_filename is not None:
+        src_with_stdin_filename = tuple(stdin_filename if s == "-" else s for s in src)
+    else:
+        src_with_stdin_filename = src
+    root, method = (
+        find_project_root(src_with_stdin_filename) if code is None else (None, None)
+    )
     ctx.obj["root"] = root
 
     if verbose:
@@ -480,7 +486,9 @@ def main(  # noqa: C901
             )
 
             normalized = [
-                (normalize_path_maybe_ignore(Path(source), root), source)
+                (source, source)
+                if source == "-"
+                else (normalize_path_maybe_ignore(Path(source), root), source)
                 for source in src
             ]
             srcs_string = ", ".join(