Building Persistent UIs with the Chromium Side Panel API

This article explains how the Chromium Side Panel API enables developers to render persistent companion user interfaces inside the browser. It examines the architectural advantages of the side panel over traditional extension views, the required manifest configurations, state management across different browser tabs, and the runtime methods used to control and synchronize the interface dynamically.

Architectural Advantages Over Traditional Views

Historically, Chromium extensions relied on toolbar popups or injected content scripts to display user interfaces. Both approaches have limitations regarding persistence:

  • Popups are ephemeral; they immediately close and lose their execution state the moment a user clicks back into the host webpage.
  • Content Scripts must be injected directly into the host page's Document Object Model (DOM). They disappear whenever the user navigates to a new URL and are vulnerable to styling conflicts with the host website's CSS.

The Side Panel API solves these issues by allocating a dedicated, browser-managed viewport directly alongside the primary web content. Because this pane is hosted by the browser shell rather than the webpage, the extension's user interface remains open, retains its state, and continues running as the user navigates across different URLs or switches between tabs.

Manifest Configuration and Execution Context

To utilize the Side Panel API, an extension must use Manifest V3 and declare the "sidePanel" permission within its manifest.json file. Developers specify the entry point for the interface by defining an HTML file inside the "side_panel" manifest key:

{
  "name": "Companion UI Extension",
  "version": "1.0",
  "manifest_version": 3,
  "permissions": [
    "sidePanel"
  ],
  "side_panel": {
    "default_path": "panel.html"
  }
}

When rendered, panel.html runs in its own isolated extension context, identical to how a background page or popup operates. This isolation guarantees that the companion UI is completely immune to the host webpage's scripts and styles, while granting the side panel's JavaScript direct access to all permitted Chromium extension APIs.

Global vs. Tab-Specific Persistence

The Side Panel API gives developers granular control over how the interface persists across the browser using the chrome.sidePanel.setOptions() method. The API supports two primary models:

  1. Global Persistence: By default, defining a default_path in the manifest creates a global side panel. A single instance of the panel remains open and unchanged regardless of which tab the user selects. This is ideal for tools like persistent note-takers, media players, or AI chat assistants.
  2. Tab-Specific Persistence: Extensions can bind a side panel to a specific tab by passing a tabId parameter to chrome.sidePanel.setOptions(). When configured this way, the browser automatically hides the panel when the user switches to a different tab and restores it when the user returns. Developers can also load entirely different HTML files for different tabs, or disable the panel on specific websites by setting the enabled property to false.

Triggering the Side Panel

Chromium provides streamlined methods for opening the side panel without requiring users to navigate complex browser menus:

  • Toolbar Action Integration: By calling chrome.sidePanel.setPanelBehavior({ openPanelOnActionClick: true }) inside the extension's service worker, clicking the extension's toolbar icon will automatically toggle the side panel open or closed.
  • Programmatic Opening: Developers can force the side panel to open using the chrome.sidePanel.open() method. To prevent abuse, Chromium restricts this method so that it can only be executed in direct response to a user gesture, such as clicking a context menu item, pressing a keyboard shortcut, or interacting with an element injected into the webpage.

State Synchronization and Communication

Because a persistent side panel remains active while the main browser viewport changes, the panel must actively listen for browser events to stay relevant to the user's current task.

The side panel's JavaScript can monitor tab navigation and switching by registering listeners for chrome.tabs.onActivated and chrome.tabs.onUpdated. To interact directly with the webpage the user is viewing, the side panel uses standard Chromium message-passing APIs (chrome.runtime.sendMessage and chrome.tabs.sendMessage). This allows the persistent companion UI to continuously extract data from, or trigger changes within, the active tab's DOM via content scripts without ever reloading its own interface.