Commits on Feb 24, 2016

1. compositing: Stop compositing unnecessarily after each animation frame. …

   Instead, schedule a delayed composite after each frame of an animation.
   
   The previous code would cause jank, because the following sequence
   frequently occurred:
   
   1. The page uses `requestAnimationFrame()` to request a frame.
   
   2. The compositor receives the message, schedules a composite,
   dispatches the rAF message to the script thread, composites, and goes to
   sleep waiting for vblank (frame 1).
   
   3. The script makes a change and sends it through the pipeline.
   Eventually it gets painted and is sent to the compositor, but the
   compositor is sleeping.
   
   4. The compositor wakes up, sees the new painted content, page flips,
   and goes to sleep (frame 2). Repeat from step 1.
   
   The problem is that we have two composition frames, not just one. This
   halves Web apps' framerate!
   
   This commit fixes the problem by scheduling the composite in step 2 to
   12 ms in the future. We already have this delayed-composition
   functionality in the form of the scrolling timer, which I repurposed and
   renamed to the "delayed composition timer" for this task. This change
   gives the page 12 ms to prepare the frame, which seems to usually be
   enough, especially with WebRender.
   
   Note that simply removing the scheduled composite after rAF is not the
   correct solution. If this is done, then pages that call rAF and don't
   modify the page won't receive future rAFs, since the compositor will be
   sleeping and won't be notified of vblank.
   
   Fixes a bunch of jank in browser.html. The remaining jank seems to be a
   problem with browser.html itself.

   

   pcwalton committed Feb 24, 2016

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