diff --git a/lib/__tests__/demo-client-files.test.ts b/lib/__tests__/demo-client-files.test.ts
new file mode 100644
index 000000000..9a4c9b873
--- /dev/null
+++ b/lib/__tests__/demo-client-files.test.ts
@@ -0,0 +1,22 @@
+import { describe, expect, it } from 'vitest';
+import { DemoJMAPClient } from '@/lib/demo/demo-client';
+
+// A move, its Undo and a restore from the trash are all parentId updates;
+// none of them may make a file look edited.
+describe('DemoJMAPClient files', () => {
+  it('keeps a file\'s Modified time when it is moved or renamed', async () => {
+    const client = new DemoJMAPClient();
+    const folder = await client.createFileDirectory('Target', null);
+    const nodes = await client.listAllFileNodes();
+    const original = nodes.find((n) => n.blobId != null && n.modified)!;
+    const before = original.modified;
+
+    await client.updateFileNodes({ [original.id]: { parentId: folder.id } });
+    await client.updateFileNode(original.id, { name: 'renamed.txt' });
+
+    const after = (await client.getFileNodes([original.id]))[0];
+    expect(after.parentId).toBe(folder.id);
+    expect(after.name).toBe('renamed.txt');
+    expect(after.modified).toBe(before);
+  });
+});
diff --git a/lib/demo/demo-client.ts b/lib/demo/demo-client.ts
index 44bb9e3d7..f1790676e 100644
--- a/lib/demo/demo-client.ts
+++ b/lib/demo/demo-client.ts
@@ -1409,8 +1409,10 @@ export class DemoJMAPClient implements IJMAPClient {
   }
 
   async updateFileNode(id: string, updates: Partial<Pick<FileNode, 'name' | 'parentId'>>): Promise<void> {
+    // A rename or move leaves `modified` alone, as on Stalwart: it dates the
+    // content, and a move or its Undo shouldn't make a file look edited.
     const node = this.data.fileNodes.find(n => n.id === id);
-    if (node) Object.assign(node, updates, { modified: new Date().toISOString() });
+    if (node) Object.assign(node, updates);
   }
 
   async updateFileNodes(updates: Record<string, Partial<Pick<FileNode, 'name' | 'parentId'>>>): Promise<{ updated: string[]; notUpdated: Record<string, string> }> {
@@ -1418,7 +1420,7 @@ export class DemoJMAPClient implements IJMAPClient {
     for (const [id, patch] of Object.entries(updates)) {
       const node = this.data.fileNodes.find(n => n.id === id);
       if (node) {
-        Object.assign(node, patch, { modified: new Date().toISOString() });
+        Object.assign(node, patch);
         updated.push(id);
       }
     }