Just mentioning an approach that is similar to what we've done in Settings to restore focus after a dialog is closed.

 - When entering details view, store the focused element.
 - When exiting details view, retrieve the focused element, and if still exists, focus it.

view-manager fires view-exit-finish, view-enter-finish events that can be leveraged in this case, see [1].

[1] https://cs.chromium.org/chromium/src/chrome/browser/resources/md_extensions/view_manager.js?sq=package:chromium&dr&l=89,69

FYI, initial CL by @dschuyler is at https://chromium-review.googlesource.com/c/chromium/src/+/773576. I continued that work at https://chromium-review.googlesource.com/c/chromium/src/+/777843 (currently in review), since dschuyler@ is OOO.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a916e01e2cca36c17ee5383b3f136bf70d19825e

commit a916e01e2cca36c17ee5383b3f136bf70d19825e
Author: dpapad <dpapad@chromium.org>
Date: Mon Nov 20 22:49:48 2017

MD Extensions: Restore focus when exiting a subpage.

Bug:  784567 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ic9a0ec829cb8c5defea347ab954dbe4c29227908
Reviewed-on: https://chromium-review.googlesource.com/777843
Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517964}
[modify] https://crrev.com/a916e01e2cca36c17ee5383b3f136bf70d19825e/chrome/browser/resources/md_extensions/item_list.html
[modify] https://crrev.com/a916e01e2cca36c17ee5383b3f136bf70d19825e/chrome/browser/resources/md_extensions/item_list.js
[modify] https://crrev.com/a916e01e2cca36c17ee5383b3f136bf70d19825e/chrome/browser/resources/md_extensions/manager.js

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/faff27431c132804997c297eab3dc7ac450cfcb9

commit faff27431c132804997c297eab3dc7ac450cfcb9
Author: Takeshi Yoshino <tyoshino@chromium.org>
Date: Tue Nov 21 04:55:32 2017

Revert "MD Extensions: Restore focus when exiting a subpage."

This reverts commit a916e01e2cca36c17ee5383b3f136bf70d19825e.

Reason for revert: Suspected to have caused a bot failure.

Original change's description:
> MD Extensions: Restore focus when exiting a subpage.
> 
> Bug:  784567 
> Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
> Change-Id: Ic9a0ec829cb8c5defea347ab954dbe4c29227908
> Reviewed-on: https://chromium-review.googlesource.com/777843
> Reviewed-by: Hector Carmona <hcarmona@chromium.org>
> Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#517964}

TBR=dpapad@chromium.org,hcarmona@chromium.org

Change-Id: I485cf634bab320411e8c0cc33715baed0363011c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug:  784567 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Reviewed-on: https://chromium-review.googlesource.com/781001
Reviewed-by: Takeshi Yoshino <tyoshino@chromium.org>
Commit-Queue: Takeshi Yoshino <tyoshino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518122}
[modify] https://crrev.com/faff27431c132804997c297eab3dc7ac450cfcb9/chrome/browser/resources/md_extensions/item_list.html
[modify] https://crrev.com/faff27431c132804997c297eab3dc7ac450cfcb9/chrome/browser/resources/md_extensions/item_list.js
[modify] https://crrev.com/faff27431c132804997c297eab3dc7ac450cfcb9/chrome/browser/resources/md_extensions/manager.js

Investigating the test failure and plan to reland once the problem has been addressed.

Found the problem and uploaded new CL with the issue fixed, see https://chromium-review.googlesource.com/c/chromium/src/+/782740/2..3.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/81c74e23a9ef607ad3598d6f243fe4786da10b86

commit 81c74e23a9ef607ad3598d6f243fe4786da10b86
Author: dpapad <dpapad@chromium.org>
Date: Wed Nov 22 01:36:24 2017

Reland "MD Extensions: Restore focus when exiting a subpage."

This is a reland of a916e01e2cca36c17ee5383b3f136bf70d19825e
with a minor typo fix which was causing the test failure.

Original change's description:
> MD Extensions: Restore focus when exiting a subpage.
>
> Bug:  784567 
> Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
> Change-Id: Ic9a0ec829cb8c5defea347ab954dbe4c29227908
> Reviewed-on: https://chromium-review.googlesource.com/777843
> Reviewed-by: Hector Carmona <hcarmona@chromium.org>
> Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#517964}

Bug:  784567 
Change-Id: Ib719bfca145ab2639358ccace41c8647a65bad5e
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Reviewed-on: https://chromium-review.googlesource.com/782740
Reviewed-by: catmullings <catmullings@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518490}
[modify] https://crrev.com/81c74e23a9ef607ad3598d6f243fe4786da10b86/chrome/browser/resources/md_extensions/item_list.html
[modify] https://crrev.com/81c74e23a9ef607ad3598d6f243fe4786da10b86/chrome/browser/resources/md_extensions/item_list.js
[modify] https://crrev.com/81c74e23a9ef607ad3598d6f243fe4786da10b86/chrome/browser/resources/md_extensions/manager.js