Using the per-revision bisect providing the bisect results,
Good build:63.0.3206.0(Revision:499528).
Bad build:63.0.3207.0(Revision:499829).

You are probably looking for a change made after 499817 (known good), but no later than 499818 (first known bad).

CHANGELOG URL:

https://chromium.googlesource.com/chromium/src/+log/269abb4c50e78fb570477022635210dbab7c25db..8b93f9a9a5a619593b1956032332e3a83a252d5d

From the CL above, assigning the issue to the concern owner

Suspect: https://chromium.googlesource.com/chromium/src/+/8b93f9a9a5a619593b1956032332e3a83a252d5d

@fsamuel : Could you please look into the issue, pardon me if it has nothing to do with your changes and if possible please assign it to concern owner.

Adding RB Label as this is a recent Regression. Please remove if not required.
Thank You.

Looping ekaramad@ as the reviewer of the CL, for more inputs.

IIUC, docs pages download a PDF in response to print and after detecting the user agent, in case of Chrome, they will attach an <embed> (to something) and end up creating a MimeHandlerViewContainer. Then, post messaging 'print' to the <embed> will fire up print preview.

Now from what I see, |guest_loaded_| is still |false| when the post message happens. Perhaps we do not even create a MimeHandlerViewGuest which means no one will ever ACK the creation for MimeHandlerViewContainer and hence the post message is never sent (sending the post message won't make sense anyway since there is no guest).

I can confirm that dropping |element_size_.IsEmpty()| from this conditional will fix the issue:
https://cs.chromium.org/chromium/src/extensions/renderer/guest_view/mime_handler_view/mime_handler_view_container.cc?rcl=94b4883e25fdfa02af37648977777ba539bde1c2&l=320
It also does not seem to be regressing  bug 761705 .

fsamuel@: Could you please confirm if there is any potential errors in doing as I suggested here (in this WIP CL: https://chromium-review.googlesource.com/c/chromium/src/+/666198)?

I can confirm that the issue is definitely with the way we handle element size here. This is the repro step I found which does not require the slides page:

1- Create an <embed> of type PDF in javascrip, e.g.,
 var e = document.createElement('embed');
 e.type = 'application/pdf';
 e.src = 'path_to_pdf';
2- Set the width and height to 0.
  e.style.width = '0px';
  e.style.height = '0px';
3- Attach the embed.
  document.body.appendChild(e);
4- Post message print:
  e.postMessage({type: 'print'}).

On stable channel, this pops up the print screen. On Canary it does not. By setting the size to 0 before attaching the element we will never create a MimeHandlerViewGuest.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3a9c415ac75740facaf590ab918d9eae5d6b2301

commit 3a9c415ac75740facaf590ab918d9eae5d6b2301
Author: EhsanK <ekaramad@chromium.org>
Date: Tue Sep 19 14:03:03 2017

Use base::Optional for both |view_rect_| and |element_size_|

This CL allows creation of MimeHandlerViewGuest even when element size is
0x0. This is specifically important for Google slides pages where an
<embed> of size 0 is used for print previewing the page. Essentially,
we need to makes sure for such <embed>'s the guest is created (after
attaching) and embed.postMessage works fine.

Bug :  763812 

Change-Id: I7ad0cd0898aee2e1b5d6b15dd9bfdcb100a8e16b
Reviewed-on: https://chromium-review.googlesource.com/667900
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Reviewed-by: Kevin McNee <mcnee@chromium.org>
Reviewed-by: Fady Samuel <fsamuel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502847}
[modify] https://crrev.com/3a9c415ac75740facaf590ab918d9eae5d6b2301/content/renderer/browser_plugin/browser_plugin.cc
[modify] https://crrev.com/3a9c415ac75740facaf590ab918d9eae5d6b2301/content/renderer/browser_plugin/browser_plugin.h
[modify] https://crrev.com/3a9c415ac75740facaf590ab918d9eae5d6b2301/extensions/renderer/guest_view/mime_handler_view/mime_handler_view_container.cc
[modify] https://crrev.com/3a9c415ac75740facaf590ab918d9eae5d6b2301/extensions/renderer/guest_view/mime_handler_view/mime_handler_view_container.h

Retested above issue in latest chrome canary #63.0.3220.0 on Windows (7,8,8.1,10),Linux (14.04 LTS), Mac (10.12.6) OS and issue seems fixed. Kindly review an attached video.

Deleted: Fixed .mov 5.8 MB

Fixed .mov 5.8 MB Download

Changing the status to 'Fixed' as the Issue is verified in C#9. Please reopen if there is still work to be done here.

Thank you!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/98ee1b0984ac7ac9b4f4513a0f54e6d7785d7418

commit 98ee1b0984ac7ac9b4f4513a0f54e6d7785d7418
Author: EhsanK <ekaramad@chromium.org>
Date: Tue Sep 26 20:34:34 2017

Add test for a recent regression in PDF Viewer on slides pages

Adding a test for https://crrev.com/502847.

Bug:  766982 ,  763812 
Change-Id: I23bd8d27cb2b83ee527f12a9103f82e8eac0b115
Reviewed-on: https://chromium-review.googlesource.com/682597
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504473}
[modify] https://crrev.com/98ee1b0984ac7ac9b4f4513a0f54e6d7785d7418/chrome/browser/pdf/pdf_extension_test.cc
[add] https://crrev.com/98ee1b0984ac7ac9b4f4513a0f54e6d7785d7418/chrome/test/data/pdf/post_message_zero_sized_embed.html