extension-ui

Extension UI

When to Use

• Building popup, options page, side panel, or devtools panel
• Choosing between React/Vue/Svelte and vanilla JS for extension UI
• Handling UI state that needs to persist across popup open/close

Core Jobs

1. UI Surface Types

Surface	File	Trigger	Size
Popup	popup.html	Toolbar icon click	800×600px max
Options page	options.html	Right-click → Options	Full browser tab
Side panel	sidepanel.html	chrome.sidePanel API	Persistent panel
DevTools panel	devtools.html	DevTools → custom tab	DevTools pane
New tab override	newtab.html	New tab	Full page

2. Popup Best Practices

• Popup closes when user clicks outside — state is LOST (design accordingly)
• Load state from chrome.storage on open; save to storage on change
• Keep popup lightweight — heavy work goes in background service worker
• Popup communicates with background via chrome.runtime.sendMessage

// popup.js
document.addEventListener('DOMContentLoaded', async () => {
  const { count } = await chrome.storage.local.get('count');
  document.getElementById('count').textContent = count ?? 0;
});

3. Options Page

// manifest.json
"options_ui": {
  "page": "options.html",
  "open_in_tab": true
}

• Full page context — no size restrictions
• Ideal for complex settings
• Use chrome.storage.sync for user preferences (syncs across devices)

4. Side Panel (Chrome 114+)

// manifest.json
"side_panel": { "default_path": "sidepanel.html" },
"permissions": ["sidePanel"]

// Open side panel on action click
chrome.sidePanel.setPanelBehavior({ openPanelOnActionClick: true });

5. Framework Setup (React + Vite)

Use Plasmo or WXT for React/Vue extensions:

# Plasmo
npm create plasmo@latest

# WXT
npm create wxt@latest

Handles: manifest generation, HMR in dev, production build, MV3 compatibility.

Vanilla JS for simple extensions — no framework overhead, faster popup load.

Key Concepts

• Popup — temporary, closes on blur, state must be persisted to storage
• Options page — persistent tab, ideal for settings
• Side panel — Chrome 114+; persistent, visible alongside page
• Plasmo/WXT — extension frameworks handling build tooling, HMR, manifest generation
• Shadow DOM — isolate injected UI from page styles

Checklist

• Popup reads state from chrome.storage on open (not relying on background state)?
• Options page uses chrome.storage.sync for user preferences?
• Side panel enabled in manifest if needed (Chrome 114+ only)?
• CSP-compliant: no inline scripts in HTML files?
• Build tool (Plasmo/WXT/Vite) configured for MV3?

Output Format

• 🔴 Critical — inline scripts in HTML (CSP violation), popup storing state in memory
• 🟡 Warning — popup doing heavy computation (should be in background), no persistence between popup opens
• 🟢 Suggestion — side panel instead of popup for complex UI, Plasmo/WXT for React apps

Common Pitfalls

• Popup "loses state" — because it's a new page on each open; always load from storage
• CSP blocks inline <script> — move all JS to external files
• window.close() in popup closes the popup, not the browser window