I bisected this to:

e616115a9b1dca27ffdf447c79b4413305c49bb4 is the first bad commit
commit e616115a9b1dca27ffdf447c79b4413305c49bb4
Author: Francois Doray <fdoray@chromium.org>
Date:   Tue Mar 27 22:05:37 2018 +0000

    Handle WebContents occlusion on aura (reland).
    
    This CL is identical to
    https://chromium-review.googlesource.com/c/chromium/src/+/937820, which
    was reverted because it broke window resizer ( https://crbug.com/818010 ).
    That issue was addressed in
    https://chromium-review.googlesource.com/c/chromium/src/+/970746.
    I also verified that this CL doesn't break Alt-Tab, overview mode,
    multi-profile and testForegroundTab test ( https://crbug.com/817957 ,
    100/100 runs pass now vs 12/100 runs without
    https://chromium-review.googlesource.com/c/chromium/src/+/970746).
    
    With this CL, when a WebContents is occluded on aura:
    - WebContents::GetVisibility returns OCCLUDED
    - RenderWidgetHostImpl::WasHidden is called on all
      RenderWidgetHostImpls that belong to the WebContents.
      That allows:
      - Changing the priority of renderer processes to background.
      - Not rendering frames unnecessarily.
    
    TBR=dominickn@chromium.org,sky@chromium.org,avi@chromium.org,peter@chromium.org
    
    Bug: 668690
    Change-Id: I040b93738561c463654c50d6f42ce19c447c6693
    Reviewed-on: https://chromium-review.googlesource.com/973682
    Commit-Queue: François Doray <fdoray@chromium.org>
    Reviewed-by: Scott Violet <sky@chromium.org>
    Cr-Commit-Position: refs/heads/master@{#546286}

Is this still an issue? Please provide an update.

This is an RBS; please revert or submit merge request for change.

Please escalate attention / fix/ merge since stable is approaching.  There hasn't been an update for the P1 RBS since April 28th; please reconsider RBS status.

I uploaded a one-line fix CL https://chromium-review.googlesource.com/c/chromium/src/+/1076349 and am waiting for review.

A lot of dependent CLs have landed, so reverting https://chromium-review.googlesource.com/973682 would be complicated.

Thanks François, can you check on the CL to push it along for submit / testing?  

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cb90179064ccb9b3586ea2de8f449b1f7c43f006

commit cb90179064ccb9b3586ea2de8f449b1f7c43f006
Author: Francois Doray <fdoray@chromium.org>
Date: Wed May 30 19:09:04 2018

Mark the WindowDimmer window as transparent.

Any window which isn't fully opaque must be marked as transparent.
Otherwise, aura::WindowOcclusionTracker will consider it fully opaque
in occlusion computations.

Bug:  833814 
Change-Id: Ie2cc66319efc3c4c0f3f014a685aa7ca2fff2ef8
Reviewed-on: https://chromium-review.googlesource.com/1076349
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562948}
[modify] https://crrev.com/cb90179064ccb9b3586ea2de8f449b1f7c43f006/ash/BUILD.gn
[modify] https://crrev.com/cb90179064ccb9b3586ea2de8f449b1f7c43f006/ash/wm/window_dimmer.cc
[add] https://crrev.com/cb90179064ccb9b3586ea2de8f449b1f7c43f006/ash/wm/window_dimmer_unittest.cc

Please submit a merge request after this change is tested on ToT.

Ping since this is schedule impacting.  Is #7 the fix, and should it be considered for merge?

Request to merge cb90179064ccb9b3586ea2de8f449b1f7c43f006 to M68 branch. This is a trivial fix for a RBS issue.

Your change meets the bar and is auto-approved for M68. Please go ahead and merge the CL to branch 3440 manually. Please contact milestone owner if you have questions.
Owners: cmasso@(Android), kariahda@(iOS), bhthompson@(ChromeOS), abdulsyed@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Merged to branch 3440 https://chromium-review.googlesource.com/c/chromium/src/+/1085027

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f62a33fe592b5490369f631cbd5dad5651811016

commit f62a33fe592b5490369f631cbd5dad5651811016
Author: Francois Doray <fdoray@chromium.org>
Date: Mon Jun 04 13:13:46 2018

[Merge M68] Mark the WindowDimmer window as transparent.

Any window which isn't fully opaque must be marked as transparent.
Otherwise, aura::WindowOcclusionTracker will consider it fully opaque
in occlusion computations.

Bug:  833814 
Change-Id: Ie2cc66319efc3c4c0f3f014a685aa7ca2fff2ef8
Reviewed-on: https://chromium-review.googlesource.com/1076349
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#562948}(cherry picked from commit cb90179064ccb9b3586ea2de8f449b1f7c43f006)
Reviewed-on: https://chromium-review.googlesource.com/1085027
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/branch-heads/3440@{#131}
Cr-Branched-From: 010ddcfda246975d194964ccf20038ebbdec6084-refs/heads/master@{#561733}
[modify] https://crrev.com/f62a33fe592b5490369f631cbd5dad5651811016/ash/BUILD.gn
[modify] https://crrev.com/f62a33fe592b5490369f631cbd5dad5651811016/ash/wm/window_dimmer.cc
[add] https://crrev.com/f62a33fe592b5490369f631cbd5dad5651811016/ash/wm/window_dimmer_unittest.cc