diff --git a/rtl-spec/components/sidebar-file-tree.spec.tsx b/rtl-spec/components/sidebar-file-tree.spec.tsx
index 922df2eb12..5b5cb07686 100644
--- a/rtl-spec/components/sidebar-file-tree.spec.tsx
+++ b/rtl-spec/components/sidebar-file-tree.spec.tsx
@@ -96,7 +96,35 @@ describe('SidebarFileTree component', () => {
     ) as HTMLInputElement;
     await user.type(input, 'tester.js{Enter}');
 
-    expect(editorMosaic.files.get('tester.js')).toBe(EditorPresence.Pending);
+    expect(editorMosaic.files.get('tester.js')).toBe(EditorPresence.Hidden);
+  });
+
+  it('fails to create new editors (file name with path separator)', async () => {
+    const user = userEvent.setup();
+    const { container } = render(<SidebarFileTree appState={store} />);
+    const EDITOR_NEW_NAME = 'subdir/tester.js';
+
+    expect(editorMosaic.files.get(EDITOR_NEW_NAME)).toBe(undefined);
+
+    // Click the "Add New File" button (by icon)
+    const addButton = container.querySelector(
+      'button .bp3-icon-add',
+    )?.parentElement;
+    await user.click(addButton!);
+
+    // Type the filename and press Enter
+    const input = container.querySelector(
+      '#new-file-input',
+    ) as HTMLInputElement;
+    await user.type(input, `${EDITOR_NEW_NAME}{Enter}`);
+
+    // Wait for error dialog to be called
+    await waitFor(() => {
+      console.log('store.showErrorDialog: ', store.showErrorDialog);
+      expect(store.showErrorDialog).toHaveBeenCalledWith(
+        `Invalid filename "${EDITOR_NEW_NAME}": filenames cannot include path separators`,
+      );
+    });
   });
 
   it('can delete editors', async () => {
@@ -237,6 +265,36 @@ describe('SidebarFileTree component', () => {
     expect(editorMosaic.files.get(EDITOR_NAME)).toBe(EditorPresence.Pending);
   });
 
+  it('fails if trying to rename an editor to an unsupported name (file name with path separator)', async () => {
+    const user = userEvent.setup();
+    const EDITOR_NAME = 'index.html';
+    const EDITOR_NEW_NAME = 'msg/data.json';
+
+    store.showInputDialog = vi.fn().mockResolvedValueOnce(EDITOR_NEW_NAME);
+    store.showErrorDialog = vi.fn().mockResolvedValueOnce(true);
+
+    const { container } = render(<SidebarFileTree appState={store} />);
+
+    // Right-click on index.html to open context menu
+    const fileLabel = Array.from(container.querySelectorAll('.pointer')).find(
+      (el) => el.textContent === EDITOR_NAME,
+    ) as HTMLElement;
+    fireEvent.contextMenu(fileLabel);
+
+    // Click the "Rename" menu item
+    const renameItem = await screen.findByText('Rename');
+    await user.click(renameItem);
+
+    // Wait for error dialog to be called
+    await waitFor(() => {
+      expect(store.showErrorDialog).toHaveBeenCalledWith(
+        `Invalid filename "${EDITOR_NEW_NAME}": filenames cannot include path separators`,
+      );
+    });
+
+    expect(editorMosaic.files.get(EDITOR_NAME)).toBe(EditorPresence.Pending);
+  });
+
   it('fails if trying to rename an editor to an existing name', async () => {
     const user = userEvent.setup();
     const EXISTED_NAME = 'styles.css';
diff --git a/src/renderer/components/sidebar-file-tree.tsx b/src/renderer/components/sidebar-file-tree.tsx
index 634b055920..d7148541e1 100644
--- a/src/renderer/components/sidebar-file-tree.tsx
+++ b/src/renderer/components/sidebar-file-tree.tsx
@@ -14,10 +14,10 @@ import { ContextMenu2, Tooltip2 } from '@blueprintjs/popover2';
 import classNames from 'classnames';
 import { observer } from 'mobx-react';
 
-import { EditorId, PACKAGE_NAME } from '../../interfaces';
+import { EditorId } from '../../interfaces';
 import { EditorPresence } from '../editor-mosaic';
 import { AppState } from '../state';
-import { isMainEntryPoint, isSupportedFile } from '../utils/editor-utils';
+import { isMainEntryPoint } from '../utils/editor-utils';
 
 interface FileTreeProps {
   appState: AppState;
@@ -108,7 +108,7 @@ export const SidebarFileTree = observer(
             <input
               className={classNames(Classes.INPUT, Classes.FILL, Classes.SMALL)}
               style={{ width: `100%`, padding: 0 }}
-              onKeyDown={(e) => {
+              onKeyDown={async (e) => {
                 if (e.key === 'Escape') {
                   e.currentTarget.blur();
                 } else if (e.key === 'Enter') {
@@ -189,29 +189,13 @@ export const SidebarFileTree = observer(
 
       if (!id) return;
 
-      if (
-        id.endsWith('.json') &&
-        [PACKAGE_NAME, 'package-lock.json'].includes(id)
-      ) {
-        await appState.showErrorDialog(
-          `Cannot add ${PACKAGE_NAME} or package-lock.json as custom files`,
-        );
-        return;
-      }
-
-      if (!isSupportedFile(id)) {
-        await appState.showErrorDialog(
-          `Invalid filename "${id}": Must be a file ending in .cjs, .js, .mjs, .html, .css, or .json`,
-        );
-        return;
-      }
-
       try {
         await appState.editorMosaic.renameFile(editorId, id);
 
         if (visible) appState.editorMosaic.show(id);
       } catch (err: any) {
-        appState.showErrorDialog(err.message);
+        await new Promise<void>((r) => setTimeout(() => r(), 100));
+        await appState.showErrorDialog(err.message);
       }
     };
 
@@ -220,13 +204,14 @@ export const SidebarFileTree = observer(
       editorMosaic.remove(editorId);
     };
 
-    public createEditor = (editorId: EditorId) => {
+    public createEditor = async (editorId: EditorId) => {
       const { appState } = this.props;
       try {
-        appState.editorMosaic.addNewFile(editorId);
-        appState.editorMosaic.show(editorId);
+        await appState.editorMosaic.addNewFile(editorId);
+        await appState.editorMosaic.show(editorId);
       } catch (err: any) {
-        appState.showErrorDialog(err.message);
+        await new Promise<void>((r) => setTimeout(() => r(), 100));
+        await appState.showErrorDialog(err.message);
       }
     };
 
diff --git a/src/renderer/editor-mosaic.ts b/src/renderer/editor-mosaic.ts
index 325196fd52..df4181d223 100644
--- a/src/renderer/editor-mosaic.ts
+++ b/src/renderer/editor-mosaic.ts
@@ -287,6 +287,18 @@ export class EditorMosaic {
       throw new Error(`Cannot add file "${id}": File already exists`);
     }
 
+    if (id.includes('/') || id.includes('\\')) {
+      throw new Error(
+        `Invalid filename "${id}": filenames cannot include path separators`,
+      );
+    }
+
+    if (!isSupportedFile(id)) {
+      throw new Error(
+        `Invalid filename "${id}": Must be a file ending in .cjs, .js, .mjs, .html, .css, or .json`,
+      );
+    }
+
     const entryPoint = this.mainEntryPointFile();
 
     if (isMainEntryPoint(id) && entryPoint) {
@@ -308,6 +320,27 @@ export class EditorMosaic {
       throw new Error(`Cannot rename file to "${newId}": File already exists`);
     }
 
+    if (newId.includes('/') || newId.includes('\\')) {
+      throw new Error(
+        `Invalid filename "${newId}": filenames cannot include path separators`,
+      );
+    }
+
+    if (
+      newId.endsWith('.json') &&
+      [PACKAGE_NAME, 'package-lock.json'].includes(newId)
+    ) {
+      throw new Error(
+        `Cannot add ${PACKAGE_NAME} or package-lock.json as custom files`,
+      );
+    }
+
+    if (!isSupportedFile(newId)) {
+      throw new Error(
+        `Invalid filename "${newId}": Must be a file ending in .cjs, .js, .mjs, .html, .css, or .json`,
+      );
+    }
+
     const entryPoint = this.mainEntryPointFile();
 
     if (isMainEntryPoint(newId) && entryPoint !== oldId) {
diff --git a/src/renderer/state.ts b/src/renderer/state.ts
index c421af435f..a2ee654b42 100644
--- a/src/renderer/state.ts
+++ b/src/renderer/state.ts
@@ -1023,14 +1023,16 @@ export class AppState {
     });
   }
 
-  public async showErrorDialog(label: string | JSX.Element): Promise<void> {
+  public showErrorDialog = async (
+    label: string | JSX.Element,
+  ): Promise<void> => {
     await this.showGenericDialog({
       label,
       ok: 'Close',
       type: GenericDialogType.warning,
       wantsInput: false,
     });
-  }
+  };
 
   /**
    * Ensure that any buffered console output is