The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f551fd9e9ac81db8d67dbb31c32aa5eb979a7b76

commit f551fd9e9ac81db8d67dbb31c32aa5eb979a7b76
Author: Noel Gordon <noel@chromium.org>
Date: Wed Aug 08 03:35:03 2018

Fix flaky ContextMenu/FilesAppBrowserTest after CL:1104058

Remove the use of two windows for the paste tests: pre-populate system
clipboard by copying 'hello.txt' there in the test preamble.

Make the query for context menu display + command options more precise
by prefixing the menu queries with a non-hidden content menu selector,
this to avoid matching command attributes of hidden menus.

The 'focus' remote command returns a boolean result: change all places
in the test code to assert the 'focus' remote command result.

Minor: fix comments where needed. Make tests that only need Drive only
load Drive in their setupAndWaitUntilReady() call (for speed).

Bug:  871771 
Change-Id: I242344b60f405984aaa667f1f4d6c1def97880d0
Reviewed-on: https://chromium-review.googlesource.com/1165270
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581462}
[modify] https://crrev.com/f551fd9e9ac81db8d67dbb31c32aa5eb979a7b76/ui/file_manager/integration_tests/file_manager/context_menu.js

and re: this part

> 2) Investigating related  issue 867974  , we noticed that the queries for the context menu item states could be made more precise, eg., by prefixing them with #file-context-menu:not([hidden]), since they can currently select state from a hidden content menu, leading to a false test positive.

Looking over the main.html cr-menus html, this appears to be a general problem now we have so many menus with the same command id names, "paste", "copy", etc

Spitting out a general bug for that, ...  issue 872119 

Another thing noticed from CL:1104058 and in fixing  issue 867974 , the fakeMouseRightClick code change [1].

Suspect is was not needed, and more a symptom of the test case not focusing the element it wants before sending a mouse click

test.util.sync.fakeMouseRightClick = function(contentWindow, targetQuery) {
 + contentWindow.document.querySelector(targetQuery).focus();
  var mouseDownEvent = new MouseEvent('mousedown', {bubbles: true, button: 2});
  if (!test.util.sync.sendEvent(contentWindow, targetQuery, mouseDownEvent)) {
    return false;
  }

Let's see if we can remove that part of the change, since no other test in the suite has needed the focus() call in test.util.sync.fakeMouseRightClick

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1104058/10/ui/file_manager/file_manager/background/js/test_util_base.js

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/521a9d2dc738bca089bca4a0f0813e8fc656c977

commit 521a9d2dc738bca089bca4a0f0813e8fc656c977
Author: Noel Gordon <noel@chromium.org>
Date: Wed Aug 08 06:34:47 2018

Remove CL:1104058 change to test.util.sync.fakeMouseRightClick

CL:1104058 added a focus() call internal to this test helper. No other
testcase in the FilesAppBrowserTest suite has ever needed it, and many
tests use test.util.sync.fakeMouseRightClick already.

Suspecting this additional focus() was a hangover to fix problems with
testcases added in CL:1104058 during development. They got fixed later
when the testcases added the correct remote calls to focus the element
before right-clicking that element, we suppose.

Anyhow, litmus test is removing the focus() call, and then running all
tests --gtest_filter="ContextMenu*" in RELEASE and DEBUG 5 times back-
to-back. Result: no problems, this line of code can away.

Test: browser_test --gtest_filter="ContextMenu/FileApps*"
Bug:  871771 
Change-Id: I20577b7cbda0b709b40ff83ec96e8be4838a36bb
Reviewed-on: https://chromium-review.googlesource.com/1166754
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581486}
[modify] https://crrev.com/521a9d2dc738bca089bca4a0f0813e8fc656c977/ui/file_manager/file_manager/background/js/test_util_base.js