Diagnosing Live Preview Update Issues in Stackblitz

Recognizable Condition

You edit a file in the Stackblitz editor, but the preview pane continues to show the old output, or it stays blank with no obvious error messages.

Cause/Diagnostic Table

Observed Symptom	Likely Cause	Where to Look
Preview shows stale code after saving	Editor autosave disabled or iframe sandbox blocking Hot Module Replacement (HMR)	Check if the file tab shows a dirty marker; open Devtools → Console for HMR warnings
Console error: Failed to load resource: net::ERR_BLOCKED_BY_CLIENT	Ad‑blocker or browser extension blocking Stackblitz’s internal dev server	Devtools → Network tab; look for blocked requests to webpack-dev-server
New npm package not reflected after adding to package.json	npm install not triggered because Stackblitz only runs install on explicit click or lockfile change	File explorer → package.json; see if the “npm install” button is highlighted
Preview remains blank, no errors	Project entry point misconfigured (missing index.html or wrong main field)	Check the public or src folder for an HTML file; inspect package.json “main”

Ordered Checks

1. Verify autosave and HMR: In the editor, make a small change and watch the file tab. If the tab does not lose its asterisk (*) after a few seconds, autosave may be off. Open Devtools → Console and look for messages like [HMR] Unable to update. If present, proceed to step 2.
2. Test for blocking extensions: Open an incognito window (or disable extensions temporarily) and reload the Stackblitz preview. If the preview updates, an extension is likely interfering. Note the extension name from the blocked request in the Network tab.
3. Confirm dependency installation: After adding a package to package.json, click the “npm install” button that appears above the file explorer, or run npm install in the Terminal pane (Terminal → New Terminal). You must have edit permission on the workspace; the operation writes to package-lock.json or yarn.lock. Risk: accidental overwrite of lockfiles if you run the command in a forked workspace you do not intend to keep.
4. Validate entry point: Ensure an index.html exists in the public folder (or the folder referenced by public in vite.config.js / webpack.config.js) and that the main field in package.json points to the correct JavaScript entry file (e.g., "main": "src/index.js"). If missing, create the file or adjust the config.

Fixes Tied to Findings

• Autosave/HMR issue: Click the file tab’s three‑dot menu and enable “Auto Save”. If the problem persists, manually trigger a preview reload with Ctrl+Shift+R (or click the refresh button in the preview pane). This forces the iframe to re‑establish the HMR socket.
• Blocking extension: Add Stackblitz (https://www.stackblitz.com) to the extension’s allowlist, or disable the offending extension while working. After changes, repeat step 1 to confirm HMR messages appear.
• Missing npm install: After clicking the UI button or running the command in the terminal, wait for the installation to finish (the terminal will show added X packages). Then preview should update automatically; if not, perform a hard reload (Ctrl+Shift+R).
• Incorrect entry point: Add or correct the index.html file with a root container (e.g., <div id="root"></div>) and ensure your JavaScript mounts the app to that container (document.getElementById('root')). Save the file; the preview should render the app.

Escalation Criteria

If after completing the checks above the preview still does not update:

1. Fork the workspace (File → Fork) to preserve your work, then repeat the steps in the fork.
2. Open the Stackblitz console (Help → Open DevTools) and look for any uncaught exceptions or WebSocket connection failures. Capture the error text.
3. Visit the Stackblitz status page (https://status.stackblitz.com) to verify there are no ongoing platform incidents.
4. If the issue persists, create a new blank project, copy your source files manually, and test whether the problem is project‑specific (e.g., a corrupted config).

Practical verification: after applying a fix, perform a hard reload (Ctrl+Shift+R) and confirm that the latest code appears instantly. Then open Devtools → Console and Network tabs; ensure no new errors and that all requests to the dev server return status 200.