The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d408c9d53637e378bf4adc4c619fd7b47078a4b6

commit d408c9d53637e378bf4adc4c619fd7b47078a4b6
Author: Sahel Sharify <sahel@chromium.org>
Date: Fri Apr 06 19:16:39 2018

Touchstart during fling sent to renderer but not to main thread.

Always handling touchstart as non-blocking during a touchscreen fling
causes the following regression:
If there are two scrollable divs one with touch-action: pan-x and
the second one with touch-action: pan-y, flinging horizontally on the
second one while the first one is flinging horizontally boosts the fling
on the first one rather than stopping it.

In this cl the touchstart is handled non blocking during a fling only
if it's touching the scrolling layer.

Bug:  828436 
Test: CompositorEventAckBrowserTest.TouchStartDuringFling
Change-Id: If391b962b3be15955f77b38b577444f8fa3f76be
Reviewed-on: https://chromium-review.googlesource.com/998872
Commit-Queue: Sahel Sharifymoghaddam <sahel@chromium.org>
Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548888}
[modify] https://crrev.com/d408c9d53637e378bf4adc4c619fd7b47078a4b6/content/browser/renderer_host/input/passthrough_touch_event_queue.cc
[modify] https://crrev.com/d408c9d53637e378bf4adc4c619fd7b47078a4b6/ui/events/blink/input_handler_proxy.cc
[modify] https://crrev.com/d408c9d53637e378bf4adc4c619fd7b47078a4b6/ui/events/blink/input_handler_proxy.h

Tested the issue on Ubuntu 17.10 using chrome version #67.0.3386.0 (build without fix) and chrome version #67.0.3393.0 (build with fix) as per comment #0 and observed that in both the cases bottom scroller moved to right, which is not as per expected.

Attached a screen cast for reference.

sahel@ - Could you please check the attached screen cast and please let us know the expected behaviour and help us in confirming the fix.

Thanks...!!

Deleted: 828436.webm 3.6 MB

	828436.webm 3.6 MB View Download

This is not the intended behavior after the fix, but I am not sure if in video touch scrolling is happening or is it wheel scrolling since I didn't see the grey touch slop area.

I tested the fix and it was working, the reporter of the bug also tested the fix and was happy with it.
xidachen@ could you please confirm if the landed cl fixes the issue or not?

I can confirm that this is fixed with touch flinging.