fix: resolve comments

src/lib/userPreferences.test.ts

@@ -0,0 +1,26 @@
+import { describe, expect, it } from "vitest";
+import { parentDirectoryOf } from "./userPreferences";
+
+describe("parentDirectoryOf", () => {
+	it("returns the directory for a POSIX path", () => {
+		expect(parentDirectoryOf("/Users/me/Movies/clip.mp4")).toBe("/Users/me/Movies");
+	});
+
+	it("returns the directory for a Windows path", () => {
+		expect(parentDirectoryOf("C:\\Users\\me\\Movies\\clip.mp4")).toBe("C:\\Users\\me\\Movies");
+	});
+
+	it("preserves the POSIX root when the file is at /", () => {
+		expect(parentDirectoryOf("/video.mp4")).toBe("/");
+	});
+
+	it("preserves the Windows drive root with its trailing separator", () => {
+		expect(parentDirectoryOf("C:\\video.mp4")).toBe("C:\\");
+		expect(parentDirectoryOf("D:/video.mp4")).toBe("D:/");
+	});
+
+	it("returns null when no separator is present", () => {
+		expect(parentDirectoryOf("video.mp4")).toBeNull();
+		expect(parentDirectoryOf("")).toBeNull();
+	});
+});

src/lib/userPreferences.ts

@@ -89,11 +89,26 @@ export function loadUserPreferences(): UserPreferences {
 /**
  * Extracts the parent directory from a saved file path. Handles both POSIX
  * and Windows separators since the path comes from the OS save dialog.
+ *
+ * Root directories are preserved with their trailing separator so that the
+ * value is still a valid directory path:
+ *   "/video.mp4"      -> "/"
+ *   "C:\\video.mp4"   -> "C:\\"
+ *
  * Returns null if no separator is found.
  */
 export function parentDirectoryOf(filePath: string): string | null {
 	const lastSep = Math.max(filePath.lastIndexOf("/"), filePath.lastIndexOf("\\"));
-	if (lastSep <= 0) return null;
+	if (lastSep < 0) return null;
+
+	// POSIX root, e.g. "/video.mp4" -> "/"
+	if (lastSep === 0) return filePath[0];
+
+	// Windows drive root, e.g. "C:\\video.mp4" -> "C:\\"
+	if (lastSep === 2 && /^[A-Za-z]:[/\\]/.test(filePath)) {
+		return filePath.slice(0, lastSep + 1);
+	}
+
 	return filePath.slice(0, lastSep);
 }