Note that this feature is now turned ON by default in Chromium master. So any Chrome Canary build newer than 82.0.4072.0 or Microsoft Edge build newer than 82.0.446.0 will have it ON be default. If you're on an older build, you can always manually flip the flag and try it out !

This is a demonstration of scrollbar performance improvements.

In Chromium, scrolling using the scrollbar traditionally used to happen on the main thread. You can verify this by doing the following:

  1. Go to about:flags.
  2. Set the flag "Compositor threaded scrollbar scrolling" to "Disabled".
  3. Restart the browser."
  4. Click on "Start animation." (Note that this is just for visual confirmation that the main thread is hanging).
  5. Click on "Induce main thread jank."
  6. Click on the scrollbar thumb and drag it.

Notice that the scrolling is "janky" because the main thread is intermittently hanging.

Next, change the flag to "Enabled" (or to "Default" if you're on a build newer than Chrome 82.0.4072.0 or newer than Edge 82.0.446.0) and repeat steps 3 through 6, you'll notice that scrolling is much smoother because it now happens on the compositor thread.

Intent to implement.
Design doc.