fix(frontend): preserve empty file preview edits

Treat empty contents as valid so edits to an initially empty text file
survive file switches, and use a nullish fallback when restoring a saved
draft so a cleared file is not overwritten by stale server contents.
Correct the max_bytes docstring to match its passthrough behavior.

Author: kirangadhave

frontend/src/components/editor/file-tree/__tests__/file-viewer.test.tsx

@@ -1,6 +1,6 @@
 /* Copyright 2026 Marimo. All rights reserved. */
 
-import { render, screen, waitFor } from "@testing-library/react";
+import { fireEvent, render, screen, waitFor } from "@testing-library/react";
 import { Provider } from "jotai";
 import type React from "react";
 import { beforeEach, describe, expect, it, vi } from "vitest";
@@ -12,7 +12,19 @@ import { store } from "@/core/state/jotai";
 import { FileViewer, MAX_FILE_PREVIEW_BYTES } from "../file-viewer";
 
 vi.mock("@/plugins/impl/code/LazyAnyLanguageCodeMirror", () => ({
-  LazyAnyLanguageCodeMirror: () => <div data-testid="code-editor" />,
+  LazyAnyLanguageCodeMirror: ({
+    value,
+    onChange,
+  }: {
+    value?: string;
+    onChange?: (value: string) => void;
+  }) => (
+    <textarea
+      data-testid="code-editor"
+      value={value ?? ""}
+      onChange={(evt) => onChange?.(evt.target.value)}
+    />
+  ),
 }));
 
 vi.mock("@/core/wasm/utils", () => ({ isWasm: () => false }));
@@ -93,4 +105,46 @@ describe("FileViewer bounded previews", () => {
     });
     expect(screen.queryByText(/too large to preview/i)).not.toBeInTheDocument();
   });
+
+  it("preserves edits to an initially empty text file across remount", async () => {
+    const emptyFile: FileInfo = {
+      id: "/workspace/empty.txt",
+      path: "/workspace/empty.txt",
+      name: "empty.txt",
+      isDirectory: false,
+      isMarimoFile: false,
+      size: 0,
+    };
+    const response: FileDetailsResponse = {
+      file: emptyFile,
+      contents: "",
+      mimeType: "text/plain",
+      isBase64: false,
+      isTooLarge: false,
+    };
+    const client = MockRequestClient.create({
+      sendFileDetails: vi.fn().mockResolvedValue(response),
+    });
+    store.set(requestClientAtom, client);
+
+    const { unmount } = render(
+      <FileViewer file={emptyFile} onOpenNotebook={vi.fn()} />,
+      { wrapper },
+    );
+
+    const editor =
+      await screen.findByTestId<HTMLTextAreaElement>("code-editor");
+    fireEvent.change(editor, { target: { value: "draft" } });
+    unmount();
+
+    render(<FileViewer file={emptyFile} onOpenNotebook={vi.fn()} />, {
+      wrapper,
+    });
+
+    const reopened =
+      await screen.findByTestId<HTMLTextAreaElement>("code-editor");
+    await waitFor(() => {
+      expect(reopened.value).toBe("draft");
+    });
+  });
 });

frontend/src/components/editor/file-tree/file-viewer.tsx

@@ -57,7 +57,7 @@ export const FileViewer: React.FC<Props> = ({ file, onOpenNotebook }) => {
       const mode = getFileRenderMode(details.mimeType, details.isBase64);
       if (!details.isTooLarge && mode === "text") {
         const contents = details.contents || "";
-        setInternalValue(unsavedContentsForFile.get(file.path) || contents);
+        setInternalValue(unsavedContentsForFile.get(file.path) ?? contents);
       }
       return details;
     }, [file.path]);
@@ -87,7 +87,7 @@ export const FileViewer: React.FC<Props> = ({ file, onOpenNotebook }) => {
   useEffect(() => {
     return () => {
       if (
-        !data?.contents ||
+        data?.contents == null ||
         data.isTooLarge ||
         getFileRenderMode(data.mimeType, data.isBase64) !== "text"
       ) {

marimo/_server/models/files.py

@@ -32,7 +32,7 @@ class FileListRequest(msgspec.Struct, rename="camel"):
 class FileDetailsRequest(msgspec.Struct, rename="camel"):
     # The path of the file or directory
     path: str
-    # Optional caller limit; the HTTP endpoint clamps this to its hard ceiling.
+    # Optional cap on bytes read from disk; unbounded when omitted.
     max_bytes: int | None = None