Findit identified the culprit r574150 with confidence 70.0% in the config "chromium.mac / Mac10.10 Tests"
based on the flakiness trend:

https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVy1wELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCKgAWNocm9taXVtLm1hYy9NYWMxMC4xMCBUZXN0cy8zMzczMC9pbnRlcmFjdGl2ZV91aV90ZXN0cyBvbiBNYWMtMTAuMTAvVkdGaVJISmhaMmRwYm1jdlJHVjBZV05vVkc5Q2NtOTNjMlZ5VkdGaVJISmhaME52Ym5SeWIyeHNaWEpVWlhOMExrUmxkR0ZqYUZSdlQzZHVWMmx1Wkc5M0x6QT0MCxITTWFzdGVyRmxha2VBbmFseXNpcxgBDA

If the culprit above is wrong, please file a bug using this link and hit submit:
https://bugs.chromium.org/p/chromium/issues/entry?status=Unconfirmed&labels=Pri-1,Test-Findit-Wrong&components=Tools%3ETest%3EFindit%3EFlakiness&summary=%5BFindit%5D%20Flake%20Analyzer%20-%20Wrong%20result%20for%20TabDragging/DetachToBrowserTabDragControllerTest.DetachToOwnWindow/0&comment=Link%20to%20Analysis%3A%20https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVy1wELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCKgAWNocm9taXVtLm1hYy9NYWMxMC4xMCBUZXN0cy8zMzczMC9pbnRlcmFjdGl2ZV91aV90ZXN0cyBvbiBNYWMtMTAuMTAvVkdGaVJISmhaMmRwYm1jdlJHVjBZV05vVkc5Q2NtOTNjMlZ5VkdGaVJISmhaME52Ym5SeWIyeHNaWEpVWlhOMExrUmxkR0ZqYUZSdlQzZHVWMmx1Wkc5M0x6QT0MCxITTWFzdGVyRmxha2VBbmFseXNpcxgBDA

Automatically posted by the findit-for-me app (https://goo.gl/Ot9f7N).

https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac10.10%20Tests

Sample failure: https://logs.chromium.org/v/?s=chromium%2Fbuildbucket%2Fcr-buildbucket.appspot.com%2F8941256373905402496%2F%2B%2Fsteps%2Finteractive_ui_tests_on_Mac-10.10%2F0%2Flogs%2FTabDragging__x2f_DetachToBrowserTabDragControllerTest.DetachToOwnWindow__x2f_0%2F0

Erik, could you see if this is related to some of the work you did in the test earlier? (context: https://chromium-review.googlesource.com/c/chromium/src/+/1132106)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b20bb048913985eae71b480db2a2017173ef73a2

commit b20bb048913985eae71b480db2a2017173ef73a2
Author: erikchen <erikchen@chromium.org>
Date: Thu Jul 12 20:16:25 2018

Remove assertion from DetachToOwnWindow on macOS 10.10.

On macOS 10.10, AppKit appears to use the test's generated events to
asynchronously reactivate the first window. There's no way to tell when this
asynchronous reactivation will happen, so just avoid checking the assertion in
the test. This problem appears constrained to how we generate test events, and
not to the actual behavior of Chrome.

Bug:  862859 
Change-Id: Ibab56946bef3aea9aac3123d3d8aefdb7974d063
Reviewed-on: https://chromium-review.googlesource.com/1135305
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574699}
[modify] https://crrev.com/b20bb048913985eae71b480db2a2017173ef73a2/chrome/browser/ui/views/tabs/tab_drag_controller_interactive_uitest.cc

is this fixed?

 Issue 883205  has been merged into this issue.

Should be fixed. Note that this was macOS only