+msw, fsamuel - mus related? event targeting?

Kyle, I presume this is an image you built yourself? Mus is turned on in developer builds, though not official ones, via a field trial. Can you disabling #mus in about:flags?

Disabling the flag fixed the issue, so it looks like it was due to the #mus flag.

We should probably disable the #mus flag until the issue is fixed. I'd say this should still be a P1 until that occurs.

Assigning to jamescook@ for now; reassign if needed :)

I haven't tested on a device, but this doesn't repro on linux-chromeos with ToT @ #553908 or #554177.
It'd be nice to bisect and find the cause -> khorimoto@ what git position did you build Chrome from?
And yeah, we should probably disable #mus on ToT dev builds -> Scott, WDYT?

sky@: I haven't tested this on linux-chromeos, so it's possible that this only repros on a real device. I'm currently synced to 9e6579c49468b224cb9351799b96d3090250bd75, which can reproduce the issue.

msw, do you have time to look into this? If not, pass back to me (or ping me on IM). I'm at home and don't have a dev device right now.

I think we should disable mus while sorting this out, since it will impact cros UI devs working on device.

I'm also working from home today, so I can try to repro and bisect tomorrow.
I guess we can probably disable mus on ToT dev builds sooner, I just need to figure out how :)

rkaplow@: would reverting https://chromium-review.googlesource.com/c/chromium/src/+/970913 disable #mus on ToT dev builds?
I'm not familiar with the field trial code... we want the default #mus about:flag behavior to be off on ToT builds.

khorimoto@, can you help us find someone else to confirm the issue?
I'd rather not speculatively revert the field trial unless it's confirmed and urgent.

Reverting the CL in #8 causes my ToT dev build to go from ash::Config::MUS to ash::Config::CLASSIC on linux-chromeos.

I'm dry-running a revert at https://chromium-review.googlesource.com/c/chromium/src/+/1031253 just to see if the bots still pass in classic.

That said, there's a workaround (disable mus in about:flags), so this can probably wait until tomorrow.

+jhawkins, who also saw this issue on Eve.

That being said, I think it's fine to wait for tomorrow morning to revert the patch for now if that helps you be more confident in the fix :)

I just loaded #554177 onto my Eve and Link devices using go/simplechrome.
Eve definitely repros this issue, but it seems like Link does not, I wonder why.
James is going to disable #mus on dev builds and bisect to find the cause.

Discussed with msw and sky, we're going to disable mus while I investigate this issue. The revert from comment 11 is in the CQ. I'm starting a bisect.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2a40a234f5865e534f635713c2ed7388b8f849ae

commit 2a40a234f5865e534f635713c2ed7388b8f849ae
Author: James Cook <jamescook@chromium.org>
Date: Fri Apr 27 15:48:11 2018

Revert "reland attempt 4: Adds WindowService-ChromeOS fieldtrial"

This reverts commit 1d768042590f803064afd52f0216c050715bf570.

Reason for revert: Cannot click/scroll in web contents or browser UI.
See  crbug.com/837359 

Original change's description:
> reland attempt 4: Adds WindowService-ChromeOS fieldtrial
> 
> More bugs fixed. Landing again.
> 
> I'm not sure on the name here. I went with WindowService-ChromeOS as
> at some point we will launch the WindowService on other platforms, but
> that won't be anytime soon and is likely to be very different. So, I'm
> thinking later on we have the field trial WindowService-Desktop.
> 
> BUG=814898
> TEST=none
> TBR=rkaplow@chromium.org
> 
> Change-Id: I474b97bc4e8d5904aa07b8de67e1b961adbac1f7
> Reviewed-on: https://chromium-review.googlesource.com/970913
> Reviewed-by: Scott Violet <sky@chromium.org>
> Commit-Queue: Scott Violet <sky@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#544415}

TBR=sky@chromium.org,rkaplow@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 814898
Change-Id: I619d6424676bd6ede21f5f9bed55b9adc283455f
Reviewed-on: https://chromium-review.googlesource.com/1031253
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554396}
[modify] https://crrev.com/2a40a234f5865e534f635713c2ed7388b8f849ae/testing/variations/fieldtrial_testing_config.json

This problem is caused by https://chromium-review.googlesource.com/c/chromium/src/+/1025633 ("exo: Fix event dispatch to frame view.") landed April 24.

Talked to sky. I'm not going to revert that CL yet. We're going to leave mus disabled for now.