Narrow Bisect : 
https://chromium.googlesource.com/chromium/src/+log/34d768eb8c2c2bdd8e9cd259a0661b31d5f4c303..6a8a55c8d9a949f974a30b55f7c500281a9d12bb?pretty=fuller&n=10000

Suspecting : r384869  from narrow bisect

It seems my change exposed an existing problem of print preview.  It has hidden, but focusable, INPUT elements with class=margin-control-textbox.
I'm not sure what's the intention of the INPUT element.

tkent@ - I don't think it's kosher to just slough off a p1 regression that resulted from a change you landed.

thestig@ - any ideas who should look at this?

Add more print_preview people.
I'm happy to fix this if I understand the intention of these class=margin-control-textbox elements.

ping print_preview owners
Why class=margin-control-textbox is focusable?

The textbox is focusable because the user can manually edit the value after selecting "Custom" margins (see screenshot).

I can reproduce the problem even when the textbox is not hidden (by turning on "Custom" margins and repeating the rerpo step number 3, so making the textbox not focusable when hidden does not seem sufficient anyway.

Deleted: focusable_textbox.png 80.5 KB

	focusable_textbox.png 80.5 KB View Download

Also the repro steps can be simplified.

1. Launch chrome, go to any page (no need to select any text).
2. Now press Ctrl+P and click zoom until only part of the first page is visible.
3. Now hit Tab key 4-5 time and observe.

#7, thank you for the information.  I understand the situation.

Blink scrolls <div id=preview-area> to show a focused margin-control-textbox, but overflowed area including the margin-control-textbox is not drawn because the div has overflow:hidden.  The div is not intended to be scrolled.

Margin-control-textboxes should be descendants of the scrollable area.  But the scrollable area is an <iframe> hosting the plugin. We can't move margin-control-textboxes into it.
I guess changing the scrollable area from the <iframe> to another element needs a huge change.

So I think a short-term fix would be:
 - Add focus handler to margin-control-textboxes, and it resets scroll position of <div id=preview-area>, AND
 - Make maargin-control-textboxes unfocusable if not in custom margin mode.

Focused margin-control-textbox would be invisible as ever.

I made a quick workaround.
https://codereview.chromium.org/1917413002

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/dedbae45b850e09e8de724bc678cbec27a7ed6c8

commit dedbae45b850e09e8de724bc678cbec27a7ed6c8
Author: tkent <tkent@chromium.org>
Date: Mon May 02 08:18:20 2016

print_preview: Workaround of focus-scroll issue.

* Make margin-control-textbox elements unfocusable if not in custom margin mode.

* Reset the scroll position of preview-area when margin-control-textbox is
  focused.

BUG= 601341 
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/1917413002
Cr-Commit-Position: refs/heads/master@{#390902}

[modify] https://crrev.com/dedbae45b850e09e8de724bc678cbec27a7ed6c8/chrome/browser/resources/print_preview/previewarea/margin_control.js

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

Please merge your change to M51 branch 2704 by 5:00 PM PST Friday (05/06) so we can take it for next week beta release.Thank you.

@tkent: Are you planning to merge this?

I was ooo.  Will merge the fix soon.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9e9ef8f9049d13aea9310d16ecc145b24becbc3b

commit 9e9ef8f9049d13aea9310d16ecc145b24becbc3b
Author: Kent Tamura <tkent@chromium.org>
Date: Thu May 05 23:21:59 2016

Merge "print_preview: Workaround of focus-scroll issue." to M51

* Make margin-control-textbox elements unfocusable if not in custom margin mode.

* Reset the scroll position of preview-area when margin-control-textbox is
  focused.

BUG= 601341 
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/1917413002
Cr-Commit-Position: refs/heads/master@{#390902}
(cherry picked from commit dedbae45b850e09e8de724bc678cbec27a7ed6c8)

Review URL: https://codereview.chromium.org/1954763003 .

Cr-Commit-Position: refs/branch-heads/2704@{#403}
Cr-Branched-From: 6e53600def8f60d8c632fadc70d7c1939ccea347-refs/heads/master@{#386251}

[modify] https://crrev.com/9e9ef8f9049d13aea9310d16ecc145b24becbc3b/chrome/browser/resources/print_preview/previewarea/margin_control.js