Can you clarify what the repro steps are?  Today, you can't use the back/forward buttons when a dialog is showing, since it's modal.  I'm guessing that's no longer the case in your mode?  (Can you let me know how to test it?)

If you need to force auto-dismissing dialogs on, you can do "--enable-features=AutoDismissingDialogs" from the commandline, but it's turned on with Finch in M56-.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/24a1bf068b07d183d456afdb8629e851467829c3

commit 24a1bf068b07d183d456afdb8629e851467829c3
Author: avi <avi@chromium.org>
Date: Fri Dec 02 17:21:48 2016

Allow navigation while a JavaScript dialog is up.

(It will cancel the dialog.)

BUG= 669996 
TEST=as in bug

Review-Url: https://codereview.chromium.org/2541173003
Cr-Commit-Position: refs/heads/master@{#435963}

[add] https://crrev.com/24a1bf068b07d183d456afdb8629e851467829c3/chrome/browser/ui/javascript_dialogs/javascript_dialog_browsertest.cc
[modify] https://crrev.com/24a1bf068b07d183d456afdb8629e851467829c3/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc
[modify] https://crrev.com/24a1bf068b07d183d456afdb8629e851467829c3/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.h
[modify] https://crrev.com/24a1bf068b07d183d456afdb8629e851467829c3/chrome/test/BUILD.gn
[modify] https://crrev.com/24a1bf068b07d183d456afdb8629e851467829c3/tools/metrics/histograms/histograms.xml

This will need an M56 merge.

Your change meets the bar and is auto-approved for M56 (branch: 2924)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/224e73fccf2bace546537af107f432a4725eaf50

commit 224e73fccf2bace546537af107f432a4725eaf50
Author: Avi Drissman <avi@chromium.org>
Date: Sat Dec 03 20:21:30 2016

Allow navigation while a JavaScript dialog is up.

(It will cancel the dialog.)

BUG= 669996 
TEST=as in bug

Review-Url: https://codereview.chromium.org/2541173003
Cr-Commit-Position: refs/heads/master@{#435963}
(cherry picked from commit 24a1bf068b07d183d456afdb8629e851467829c3)

Review URL: https://codereview.chromium.org/2553463002 .

Cr-Commit-Position: refs/branch-heads/2924@{#319}
Cr-Branched-From: 3a87aecc31cd1ffe751dd72c04e5a96a1fc8108a-refs/heads/master@{#433059}

[add] https://crrev.com/224e73fccf2bace546537af107f432a4725eaf50/chrome/browser/ui/javascript_dialogs/javascript_dialog_browsertest.cc
[modify] https://crrev.com/224e73fccf2bace546537af107f432a4725eaf50/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc
[modify] https://crrev.com/224e73fccf2bace546537af107f432a4725eaf50/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.h
[modify] https://crrev.com/224e73fccf2bace546537af107f432a4725eaf50/chrome/test/BUILD.gn
[modify] https://crrev.com/224e73fccf2bace546537af107f432a4725eaf50/tools/metrics/histograms/histograms.xml