This seems partially related to  issue 812348 , in that they seem to coincide.

Adding devtools component. +pfeldman: when exactly do Page Enable/Disable events come in from devtools?

I was able to repro this on usatoday after reloading a few times. It appears my guess in the description is correct, and we're getting a notification that the ChromeDevToolsManagerDelegate::ClientDetached, even though the devtools window is still open.

The ClientDetached looks like it's coming from a child frame being removed. Maybe we need to only look at main frames? Not sure.

Pavel: can someone on the devtools team lend a hand here? This unreliability makes the "block ads" checkbox very unpredictable.

I've debugged this down to site isolation, because usatoday hosts a cross-process subframe.

My guess is that we create a new PageHandler for the hosted subframe, so we need to filter those out for ad blocking (which only cares about main frames).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c84985897c2695ca31245d44c30e92abcbbefa45

commit c84985897c2695ca31245d44c30e92abcbbefa45
Author: Charles Harrison <csharrison@chromium.org>
Date: Thu Mar 01 16:51:57 2018

[subresource_filter] Fix devtools toggle with site isolation

Multiple PageHandlers can be created with isolating subframes. This
breaks the devtools toggle which expects a single PageHandler for the
main frame.

This CL adds a bit to the PageHandler which classifies whether it is
for the main frame. If it is not, then it cannot change the filtering
toggle.

Bug:  813197 
Change-Id: I7698985822f5ee159399bd1d37884436c5278d0b
Reviewed-on: https://chromium-review.googlesource.com/939602
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540185}
[modify] https://crrev.com/c84985897c2695ca31245d44c30e92abcbbefa45/chrome/browser/devtools/chrome_devtools_session.cc
[modify] https://crrev.com/c84985897c2695ca31245d44c30e92abcbbefa45/third_party/WebKit/Source/devtools/front_end/inspector_main/InspectorMain.js

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/11d9d45132c92f31beadb7f57a3028dc7cbf85cf

commit 11d9d45132c92f31beadb7f57a3028dc7cbf85cf
Author: Charles Harrison <csharrison@chromium.org>
Date: Wed Mar 07 17:47:25 2018

Add test for r540185 (Fix ad block devtools toggle with site isolation)

Bug:  813197 
Change-Id: I2cc1f37e67911bbcfacbb0235c7330b51c5146c1
Reviewed-on: https://chromium-review.googlesource.com/943741
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541491}
[modify] https://crrev.com/11d9d45132c92f31beadb7f57a3028dc7cbf85cf/chrome/browser/subresource_filter/subresource_filter_devtools_browsertest.cc