The mouse wheel events are tracked per page/frame not per individual scroller. I guess we could try tracking this per layer but I'm not sure this is worth it. flackr@ did this come up in a real world scenario?

Most of this jank will be fixed when we do async wheel events for latched scrollers. It could happen at the beginning but not during a latched scroll.

It came up on this nice Edge blog post about asynchronous scrolling (see the second browser comparison table). We already track element specific rects for touches right? Seems like wheel could just add to these rects.

https://blogs.windows.com/msedgedev/2017/03/08/scrolling-on-the-web/#vChtpkLbsj2PJwkr.97

The touch logic is pretty hairy. Thus far, we've been assuming we'll fix this with SPv2.

Well you'd need a separate list. You don't want to block touch start on a wheel listener. Ideally we wanted to get away from those lists and list properties on the layers themselves.

wjmaclean@ you were mentioning something a month or so ago about doing something for wheel events for OOPIF? Could this be related?

I don't think so. My change landed March 13th on dev.

https://codereview.chromium.org/2743223003/

This change adds a nonFastScrollableRegion for the entire root-layer of the *iFrame* which does force wheels to go via the main thread, but only within the OOPIF's renderer. It wouldn't affect the remote main-frame in a multi-renderer case.

In a single-renderer case (i.e. no OOPIF) this should have no effect; and as best I can tell the page in the description doesn't create any OOPIFs, even when --site-per-process (or --top-document-isolation) is enabled.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3e3a37a78aa5eb7ff2da5adae3fa44efccf5773c

commit 3e3a37a78aa5eb7ff2da5adae3fa44efccf5773c
Author: sunxd <sunxd@chromium.org>
Date: Fri Jul 06 22:58:43 2018

Add wheel event handler region in cc

Wheel event handlers currently make scrolling block on main thread, this
is also true even if the wheel scroll happens outside the event handler
region.

In order to make scrolling faster when the event is outside the handler,
compositor needs to store information about the regions of handlers.
This CL adds those regions to cc::Layers.

Bug: 700075
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: I67fc2a08dcab7a9d6cc7054f673ed90b79da70e1
Reviewed-on: https://chromium-review.googlesource.com/893432
Commit-Queue: Xianda Sun <sunxd@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573106}
[modify] https://crrev.com/3e3a37a78aa5eb7ff2da5adae3fa44efccf5773c/cc/input/input_handler.h
[modify] https://crrev.com/3e3a37a78aa5eb7ff2da5adae3fa44efccf5773c/cc/layers/layer.cc
[modify] https://crrev.com/3e3a37a78aa5eb7ff2da5adae3fa44efccf5773c/cc/layers/layer_impl.cc
[modify] https://crrev.com/3e3a37a78aa5eb7ff2da5adae3fa44efccf5773c/cc/layers/layer_impl.h
[modify] https://crrev.com/3e3a37a78aa5eb7ff2da5adae3fa44efccf5773c/cc/trees/layer_tree_host.cc
[modify] https://crrev.com/3e3a37a78aa5eb7ff2da5adae3fa44efccf5773c/cc/trees/layer_tree_host_impl.cc
[modify] https://crrev.com/3e3a37a78aa5eb7ff2da5adae3fa44efccf5773c/cc/trees/layer_tree_host_impl.h
[modify] https://crrev.com/3e3a37a78aa5eb7ff2da5adae3fa44efccf5773c/cc/trees/layer_tree_host_impl_unittest.cc
[modify] https://crrev.com/3e3a37a78aa5eb7ff2da5adae3fa44efccf5773c/cc/trees/layer_tree_impl.cc
[modify] https://crrev.com/3e3a37a78aa5eb7ff2da5adae3fa44efccf5773c/cc/trees/layer_tree_impl.h
[modify] https://crrev.com/3e3a37a78aa5eb7ff2da5adae3fa44efccf5773c/ui/events/blink/input_handler_proxy.cc
[modify] https://crrev.com/3e3a37a78aa5eb7ff2da5adae3fa44efccf5773c/ui/events/blink/input_handler_proxy_unittest.cc