Able to reproduce this issue on Ubuntu 14.04 using chrome latest M59 #59.0.3071.71, This issue is not observed on Mac OS 10.12.

While trying to do Chromium bisect got all good builds.. even after increasing the bad range.
Hence, Updating manual bisect info..
Cr: https://chromium.googlesource.com/chromium/src/+log/57.0.2936.0..57.0.2937.0?pretty=fuller&n=10000

Please help in assigning it to the appropriate owner.

The initial bisect is probably wrong. I'll try again.

Bisect to https://chromium.googlesource.com/chromium/src/+log/7f3c2b8170c9058511d39135cfa1ff7cf0c9a668..443064bb9932a98ad47c1cbbf8bba2493a31f888 so it's probably r448761, but the underlying reason is not clear.

Assigning to alexmos@ for more inputs on this as per C#5. 

I've confirmed that this is due to r448761, which turned AreCrossProcessFramesPossible on by default.  Among other things, this turned on the browser process event routing logic (RenderWidgetHostInputEventRouter), and this is probably related to that.  The unnecessary background scrolling occurs only when print-previewing the PDF; it's fine with a regular web page.

Ken or Ehsan, do you know what might be causing this?  Reassigning to Ken for now, since Ken's worked on both event routing and scrolling.

For what it's worth, I actually see the new behaviour as making more sense than the old. To make it work the old way, we'd probably need to implement some sort of feature locking the mouse the the print dialog, and make sure the mouse wheels get locked too.

It also happens with OOPIFs -- if you have an OOPIF visible in the margins around a print preview, you can scroll that as well.

I think this is because the top-level renderer ignores input events when a print preview is displayed, but subframe processes don't know about it. To change this we could either propagate knowledge of the print preview to all renderers, or else stop input routing in the browser process.

I agree with comment #8, this doesn't feel like a meaningful bug. Is this worth spending time fixing?

 Issue 740902  has been merged into this issue.

Comments 8-9: Being able to scroll the page in the background doesn't seem like a problem, but can you interact with it in other ways (e.g., forms, etc)?  That might be more of a concern.

If not, I'm ok with closing this.

I think the main argument for fixing this is consistency in the UI -- from a users perspective, why can some frames or a pdf scroll while in the print preview, while other sites can't?

Also, the HTML spec says that the user agent should pause while the print dialog is displayed (https://html.spec.whatwg.org/#printing).

I think we should fix this. Doesn't have to be a P1, but it would be nice to have.

Agreed.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/738ea19831a4395bff3066a8ce994482ac8b44d6

commit 738ea19831a4395bff3066a8ce994482ac8b44d6
Author: Avi Drissman <avi@chromium.org>
Date: Wed Aug 29 20:24:16 2018

Move the public "ignore events" switch to WebContents.

All public users intend to set "ignore events" on an entire
page, but the API was on widget and so it was only ever set
on the top-level widget. Move it to WebContents to allow it
to have the desired effect.

BUG= 863582 ,  725830 

Change-Id: I435b9f93160d0d77ad1aa0919757a4acd933768e
Reviewed-on: https://chromium-review.googlesource.com/1194484
Reviewed-by: Mike Wittman <wittman@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587274}
[modify] https://crrev.com/738ea19831a4395bff3066a8ce994482ac8b44d6/chrome/browser/ui/views/login_handler_views.cc
[modify] https://crrev.com/738ea19831a4395bff3066a8ce994482ac8b44d6/components/web_modal/web_contents_modal_dialog_manager.cc
[modify] https://crrev.com/738ea19831a4395bff3066a8ce994482ac8b44d6/content/browser/renderer_host/render_view_host_impl.cc
[modify] https://crrev.com/738ea19831a4395bff3066a8ce994482ac8b44d6/content/browser/renderer_host/render_widget_host_delegate.cc
[modify] https://crrev.com/738ea19831a4395bff3066a8ce994482ac8b44d6/content/browser/renderer_host/render_widget_host_delegate.h
[modify] https://crrev.com/738ea19831a4395bff3066a8ce994482ac8b44d6/content/browser/renderer_host/render_widget_host_impl.cc
[modify] https://crrev.com/738ea19831a4395bff3066a8ce994482ac8b44d6/content/browser/renderer_host/render_widget_host_impl.h
[modify] https://crrev.com/738ea19831a4395bff3066a8ce994482ac8b44d6/content/browser/renderer_host/render_widget_host_view_aura.cc
[modify] https://crrev.com/738ea19831a4395bff3066a8ce994482ac8b44d6/content/browser/web_contents/web_contents_impl.cc
[modify] https://crrev.com/738ea19831a4395bff3066a8ce994482ac8b44d6/content/browser/web_contents/web_contents_impl.h
[modify] https://crrev.com/738ea19831a4395bff3066a8ce994482ac8b44d6/content/public/browser/render_widget_host.h
[modify] https://crrev.com/738ea19831a4395bff3066a8ce994482ac8b44d6/content/public/browser/web_contents.h

Unable to reproduce the issue on reported version 57.0.2937.0 on Ubuntu 17.10 by following below steps.
1.Launched chrome 
2.Navigated to 'http://cb.vu/unixtoolbox.pdf'.
3.Clicked on print button.
4.Scrolled outside the print preview observed that the back ground page does not get scrolled.

Attached the screencast for reference.
avi@ - Could you please check the screencast and help us in verifying the fix, let us know if anything is being missed from our end.

Thanks.!

Deleted: 725830.mp4 2.6 MB

	725830.mp4 2.6 MB View Download