Split Scroll Continuum

Firefox and Chrome Desktop 140+ WebExtension that pairs two tabs showing the same page and synchronizes scroll position so both tabs behave as a two-column reading continuum when placed side by side in browser split view.

When you scroll down in the source tab, the sibling tab jumps to approximately the next readable segment — so the two tabs together cover consecutive vertical slices of one long document rather than showing the same content twice.

Requirements

• Firefox 131 or later (desktop)
• Chrome 140 or later (desktop)
• The extension requires tabs, storage, and browser-specific menu permissions plus http://*/* and https://*/* host access

Install

Build both browser packages first:

npm run build

Load the Firefox package as a temporary extension for development:

1. Open about:debugging in Firefox
2. Click This Firefox -> Load Temporary Add-on
3. Select dist/firefox/manifest.json

Load the Chrome package unpacked:

1. Open chrome://extensions
2. Enable Developer mode
3. Click Load unpacked
4. Select dist/chrome/

How it works

Pairing: Click the extension icon in the toolbar, use the page context menu item Duplicate and pair for split reading, or press Alt+Shift+P. The extension looks for another tab in the same window with the same canonical URL, preferring a split-view mate when the browser exposes split metadata. If none exists, it duplicates the current tab and pairs with it. A tab can belong to at most one pair at a time.

Scroll sync: When you scroll the source tab, the extension computes a continuum offset (sourceScrollY + viewportHeight - 32px overlap) and applies it to the sibling tab. If you start scrolling in the sibling tab instead, ownership switches automatically. Rapid ownership switching is logged for diagnostics, but sync keeps running instead of auto-pausing.

Adaptive overlap (experimental): The popup can enable a global, persisted article-aware mode that measures a likely reading container, derives overlap from its line height, and discounts obvious sticky header/footer occlusion. This mode is conservative by design: when article detection is weak or incomplete, it falls back to the current fixed 32px overlap path.

PageUp / PageDown override: By default, when a paired tab is actively syncing, pressing PageUp or PageDown in the page behaves like pressing that key twice. The popup can disable this globally, and the setting persists across tabs and browser restarts.

Split View: If the current tab is in split view and the browser exposes split metadata, the extension prefers the split mate as the pairing candidate over other duplicate tabs.

Context menu automation: The page context menu can create the duplicate-and-pair flow in one step. Firefox and Chrome 140+ expose splitViewId for detection, but they do not expose a supported API to place a tab into the other split pane programmatically, so the extension can prefer an existing split mate but cannot force creation of the split itself.

Split View auto-pairing: When two tabs enter split view on a browser that exposes split metadata and finish loading with the same canonical URL, the extension pairs them automatically and enables scroll sync without requiring a manual popup or context-menu action.

Pausing: Use Pause sync in the popup or Alt+Shift+S to pause without breaking the pair. The popup shows [user] as the pause reason for manual pauses. Legacy [oscillation] pauses from older state are auto-resumed when the tab or window becomes active again.

Global popup settings: The popup toggles for Double PageUp / PageDown and Adaptive article overlap are extension-global settings. Changing them from any tab updates all current pairs and persists for future tabs.

Known limitations

• Only top-level window.scrollY scrolling. Custom scroll containers inside the page are not tracked.
• Cross-window pairing is not supported. Pairs are window-local.
• Zoom level differences between paired tabs are not compensated.
• Non-http(s) pages (about:, file:, view-source:, PDFs) cannot be paired. The popup shows an error for these.
• Pages that block extension content scripts (privileged pages, strict CSP) will show "Sync unavailable" in the popup after pairing.
• When paired tabs have different scrollHeight values (e.g., one tab still loading dynamic content), the formula applies with clamping. A console warning is logged. Exact continuation cannot be guaranteed until both tabs settle.
• The extension does not create or move browser split panes programmatically. It only detects split metadata and prefers an existing split mate when one is available.
• Private browsing and normal windows are handled via "incognito": "spanning". Cross-window pairing between private and normal windows is not supported.
• Chrome support is limited to desktop Chrome 140+.

Development

npm test          # run unit tests
npm run lint      # run addons-linter
npm run build     # rebuild dist/firefox/ and dist/chrome/

To run web-ext lint directly:

npm run build:firefox && npx web-ext lint --source-dir dist/firefox/

Debug output

Set localStorage.setItem('split-scroll-debug', '1') in the extension popup devtools console to enable verbose logging in popup and content-script contexts.

Page classes and expected behavior

Page class	Expected behavior
Static article (Wikipedia, long blog post)	Continuum sync works reliably. Scrolling source moves sibling to the next segment.
Documentation page (MDN, docs site)	Works the same as static article.
App page with moderate content	Works if the page uses standard document scroll. Custom scroll containers are not synced.
Infinite-scroll page	Formula applies with clamping. Sync may lag as new content loads and changes scrollHeight. A debug warning is logged when heights differ.
PDF / browser PDF viewer	Cannot be paired. Popup shows "This page cannot be paired."
about:, file:, view-source:	Cannot be paired. Popup shows "This page cannot be paired."
Page with blocked content script	Can be paired by URL, but popup shows "Sync unavailable: content script not accessible on this page."