I was not able to repro, but I suspect this is some kind of race where the browser ends up sending events to the renderer out-of-order/not synchronized with the request to lock the mouse. It could also be a synthesized event (enter/leave or maybe from touch gestures).

Adding more people who are familiar with mouse event routing on the RenderWidgetHostInputEventRouter.

FYI: https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=interactive_ui_tests&tests=WebViewPointerLockInteractiveTest.PointerLock_PointerLockLostWithFocus

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/dd6eafc0150f9b5e37eb2629a51e63b9996f38bf

commit dd6eafc0150f9b5e37eb2629a51e63b9996f38bf
Author: Greg Thompson <grt@chromium.org>
Date: Thu May 17 15:48:16 2018

Log the event type when unexpected events are seen.

This is to help diagnose a DCHECK being hit when unexpected pointer lock
mouse events arise.

BUG= 843059 
R=lfg@chromium.org

Change-Id: Ifffe9300d763135ff9b95b98e7054d6360e7ee4f
Reviewed-on: https://chromium-review.googlesource.com/1063710
Reviewed-by: Lucas Gadani <lfg@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559549}
[modify] https://crrev.com/dd6eafc0150f9b5e37eb2629a51e63b9996f38bf/third_party/blink/renderer/core/frame/web_frame_widget_base.cc

I think it's a kMouseLeave event:

[4616:4740:0517/223844.322:FATAL:web_frame_widget_base.cc(343)] Check failed: false. 4

does that help?

There's nothing more I can do to help. Please take action. You could disable the test in debug configurations, remove the NOTREACHED and add relevant handling to the default case, add a new case for kMouseLeave that handles it appropriately, etc. The NOTREACHED is clearly being reached somehow, so something must be done. Thanks.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1d1955f522e8d467ab4fe05ef6e8bf0b6932a7bc

commit 1d1955f522e8d467ab4fe05ef6e8bf0b6932a7bc
Author: Lucas Gadani <lfg@chromium.org>
Date: Wed May 23 23:02:48 2018

Speculative fix for NOTREACHED in WebFrameWidgetBase::PointerLockMouseEvent.

WebViewPointerLockInteractiveTest.PointerLock_PointerLockLostWithFocus
started failing recently, where WebFrameWidgetBase::PointerLockMouseEvent
was receiving mouse leave events unexpectedly.

This CL ensures that mouse enter/leave events don't get sent while
the mouse is locked.

Bug:  843059 
Change-Id: I05cb7d7cfba2732cb56ff1bda8901ae9999c57e5
Reviewed-on: https://chromium-review.googlesource.com/1066113
Commit-Queue: Lucas Gadani <lfg@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561298}
[modify] https://crrev.com/1d1955f522e8d467ab4fe05ef6e8bf0b6932a7bc/content/browser/renderer_host/render_widget_host_input_event_router.cc

Great! Thank you for getting to the bottom of this.