Yikes, thanks.

There are two distinct issues here, the long paints, and the fact that we aren't coalescing the wheel events correctly.

The lag that you're feeling is, I think, because we aren't coalescing the events properly.

My guess is that this is due to Dave's wheel gestures work.

I've root caused it and it is the gesture event queue that is backing up.

Because of the synthetic gesture begin and ends we are generating it causes
issues with coalescing the gesture scroll updates which doesn't happen.

Collapsing a synthetic gesture scroll begin with a synthetic gesture scroll end should be the right thing to do. I've written a patch but will add a test.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/dcd46c08887b38add468d81bf3b5bf4910d0f248

commit dcd46c08887b38add468d81bf3b5bf4910d0f248
Author: dtapuska <dtapuska@chromium.org>
Date: Mon May 02 14:52:32 2016

Trackpad scrolling could lag on MacOSX

Mouse wheel gesture scrolling with synthetic begin and ends
could saturate the gesture event queue. Ensure that
a synthetic gesture begin coalesces with a synthetic
gesture end so that the updates end up coalescing as well.
This prevents laggy scrolling on MacOSX when it takes
a long time per frame to scroll.

BUG= 607340 

Review-Url: https://codereview.chromium.org/1932363002
Cr-Commit-Position: refs/heads/master@{#390942}

[modify] https://crrev.com/dcd46c08887b38add468d81bf3b5bf4910d0f248/content/browser/renderer_host/input/gesture_event_queue.cc
[modify] https://crrev.com/dcd46c08887b38add468d81bf3b5bf4910d0f248/content/browser/renderer_host/input/gesture_event_queue.h
[modify] https://crrev.com/dcd46c08887b38add468d81bf3b5bf4910d0f248/content/browser/renderer_host/input/gesture_event_queue_unittest.cc

I'll keep this bug opened and wait and test in canary before requesting merge to 51.

Thanks for the fix, we will verify in next canary.

Your change meets the bar and is auto-approved for M51 (branch: 2704)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/84fdd1d3433ee491096cddea636f45491779483e

commit 84fdd1d3433ee491096cddea636f45491779483e
Author: Dave Tapuska <dtapuska@chromium.org>
Date: Wed May 04 12:58:29 2016

Trackpad scrolling could lag on MacOSX

Mouse wheel gesture scrolling with synthetic begin and ends
could saturate the gesture event queue. Ensure that
a synthetic gesture begin coalesces with a synthetic
gesture end so that the updates end up coalescing as well.
This prevents laggy scrolling on MacOSX when it takes
a long time per frame to scroll.

BUG= 607340 

Review-Url: https://codereview.chromium.org/1932363002
Cr-Commit-Position: refs/heads/master@{#390942}
(cherry picked from commit dcd46c08887b38add468d81bf3b5bf4910d0f248)

Review URL: https://codereview.chromium.org/1949993002 .

Cr-Commit-Position: refs/branch-heads/2704@{#369}
Cr-Branched-From: 6e53600def8f60d8c632fadc70d7c1939ccea347-refs/heads/master@{#386251}

[modify] https://crrev.com/84fdd1d3433ee491096cddea636f45491779483e/content/browser/renderer_host/input/gesture_event_queue.cc
[modify] https://crrev.com/84fdd1d3433ee491096cddea636f45491779483e/content/browser/renderer_host/input/gesture_event_queue.h
[modify] https://crrev.com/84fdd1d3433ee491096cddea636f45491779483e/content/browser/renderer_host/input/gesture_event_queue_unittest.cc

Issue 617071 has been merged into this issue.

If 617071 is the same issue, then r386251 didn't fix it, since it's included in 51.0.2704.63.

Appears 617071 is not the same issue; but was caused by scroll anchoring enabled.

it is not reproducible when it is off.