The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/874feac6c0e08aab52bd08fe33d5daee0ae07fa5

commit 874feac6c0e08aab52bd08fe33d5daee0ae07fa5
Author: dtapuska <dtapuska@chromium.org>
Date: Wed Apr 20 16:08:05 2016

PDF scrolling with wheel gestures

Scroll gestures may be generated twice, as a plugin
may 'resend' them if not fully consumed by the plugin,
and they may be generated once again inside the main
renderer host. Re-sending of gesture scroll events from
the plugin causes double scroll values in PDFs
This was more noticable on Windows which uses a larger tick counter (120) vs
53 on Linux.

Fix is to not generate gestures inside the plugin container.

BUG= 604734 

Review URL: https://codereview.chromium.org/1898333002

Cr-Commit-Position: refs/heads/master@{#388508}

[modify] https://crrev.com/874feac6c0e08aab52bd08fe33d5daee0ae07fa5/content/browser/renderer_host/input/mouse_wheel_event_queue.cc

The scrolling is fixed, but the events are still logged twice. Build 2714.

I'm not sure how you are reporting the events being logged twice. Do you have a specific example?

The events go through the pipeline in a specific fashion and it may be that they are being logged on two different instances

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

When using monitorEvents as in my original report, or by using theEmbed.addEventListener('mousewheel'), this regression causes the event handler to fire twice. I can post a specific test case if you like, but it's literally `e.addEventListener('mousewheel', e => alert(e))`.

Yes please provide an exact example.

When I loaded the following PDF; http://www.pdf995.com/samples/pdf.pdf

And Inspect the document;
add monitorEvents(plugin, "mousewheel"); to the console

I only get one event reported for each tick.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8960269d9901e0715357ac4a7d092550a6ebb39a

commit 8960269d9901e0715357ac4a7d092550a6ebb39a
Author: Dave Tapuska <dtapuska@chromium.org>
Date: Thu Apr 21 20:21:04 2016

PDF scrolling with wheel gestures

Scroll gestures may be generated twice, as a plugin
may 'resend' them if not fully consumed by the plugin,
and they may be generated once again inside the main
renderer host. Re-sending of gesture scroll events from
the plugin causes double scroll values in PDFs
This was more noticable on Windows which uses a larger tick counter (120) vs
53 on Linux.

Fix is to not generate gestures inside the plugin container.

BUG= 604734 

Review URL: https://codereview.chromium.org/1898333002

Cr-Commit-Position: refs/heads/master@{#388508}
(cherry picked from commit 874feac6c0e08aab52bd08fe33d5daee0ae07fa5)

Review URL: https://codereview.chromium.org/1911013002 .

Cr-Commit-Position: refs/branch-heads/2704@{#171}
Cr-Branched-From: 6e53600def8f60d8c632fadc70d7c1939ccea347-refs/heads/master@{#386251}

[modify] https://crrev.com/8960269d9901e0715357ac4a7d092550a6ebb39a/content/browser/renderer_host/input/mouse_wheel_event_queue.cc

Marking as fixed since the pdf is no longer scrolling 2x and cannot reproduce double event listeners.

 Issue 605122  has been merged into this issue.

OK, I had a difficult time reproducing the issue, but I think I understand what it's doing now.

First: I had turned on enable-experimental-web-platform-features, which I disabled before trying to reproduce the issue. I thought this fixed the issue, but I was misled.

Second: When pressing F12 on the page, devtools shows the URL of the PDF, and I can see the events twice.
But when I choose right click > Inspect, devtools shows the URL of the extension (mhjf...), and the events were only coming in once.

So, to recap:
1. Open the pdf http://www.pdf995.com/samples/pdf.pdf
2. Press F12 (or ctrl-shift-I, or ctrl-shift-C) to open the devtools on the PDF page itself, not the extension component.
3. monitorEvents(plugin, "mousewheel");
4. Observe double-logged events.

jleedev@ Are you able to open a separate issue on this? I'm guessing the root cause is likely different. I'd set it as Platform>DevTools initially for them to look at why the opening devtools reports the events differently.