09SoftwareFirefox / Waterfox extension · WebExtensions (MV2) · JavaScript
Screen Watcher — Web Extension
A Firefox/Waterfox extension that watches a selected region of the active tab for pixel changes and raises an alarm when movement is detected — the browser counterpart to the ScreenWatcher Android app.
Open source on GitHub
The same idea, inside a tab
Screen Watcher already exists as an Android app built on MediaProjection. This extension carries the idea into the browser: pick a region of the page you are actually looking at and get told the moment something moves inside it.
It is intentionally WebExtensions-scoped — activeTab and notifications — so it is never given permanent access to every site, and it stops watching the moment you leave the tab you started it on.
What makes it technically interesting
- Drag-select a rectangle inside the visible tab; the extension samples it once per second and diffs RGBA pixels against the previous frame
- Alarm fires when at least 10 pixels change by 10 channel units or more, with a heartbeat ping every 10 seconds
- Scoped to just the tab you activate: monitoring pauses if the tab reloads, changes origin, stops responding or is no longer active (activeTab only, no blanket site access)
- Press any key or use Stop alarm to silence and re-baseline