You are probably looking for a change made after 575689 (known good), but no later than 575690 (first known bad).
CHANGELOG URL:
The script might not always return single CL as suspect as some perf builds might get missing due to failure.

https://chromium.googlesource.com/chromium/src/+log/759983cbb3649ffcaccb528b76230b1bcb93f720..dc764bb8ffc51e8aa2d977721a6a2cfd9afa12c8

Suspect: https://chromium.googlesource.com/chromium/src/+/dc764bb8ffc51e8aa2d977721a6a2cfd9afa12c8

@weili: Could you please check whether this is caused with respect to your change, if not please help us in assigning it to the right owner.

The correct behavior would be, when the Menubar/Tabstrip/Toolbar/BMB movement happens without moving the content down. Please check Chrome in Cocoa Mode for reference, how it works correctly. (A screencast is attached.) Thanks.

Deleted: Fullscreen_Cocoa_without_content_movement.mov 2.2 MB

	Fullscreen_Cocoa_without_content_movement.mov 2.2 MB View Download

oh, got it, will fix this. Thanks dbote@ for filing and mehmet@ for explaining :)

"RBS" as this is P1 blocking MacViews launch.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1406837c85633743729c75ec61f02e45b852f1f2

commit 1406837c85633743729c75ec61f02e45b852f1f2
Author: Wei Li <weili@chromium.org>
Date: Thu Jul 19 17:10:31 2018

[MacViews] Track mouse and position content correctly in fullscreen

During full screen mode, we need to track mouse movement around top UI
to lock the system menu bar. This CL sets up the correct area for such
tracking.

Also, when system menu bar slides down in full screen, only the top UI
will appears under the menu bar while the position of web content should
not move.

BUG= 864931 , 865155 , 831219

Change-Id: Ie121e6f27dd24066c1ae1e133740f01c75f118a5
Reviewed-on: https://chromium-review.googlesource.com/1142694
Commit-Queue: Wei Li <weili@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576533}
[modify] https://crrev.com/1406837c85633743729c75ec61f02e45b852f1f2/chrome/browser/ui/views/frame/browser_non_client_frame_view_mac.mm
[modify] https://crrev.com/1406837c85633743729c75ec61f02e45b852f1f2/chrome/browser/ui/views/frame/browser_view_layout.cc

[Auto-generated comment by a script] We noticed that this issue is targeted for M-69; it appears the fix may have landed after branch point, meaning a merge might be required. Please confirm if a merge is required here - if so add Merge-Request-69 label, otherwise remove Merge-TBD label. Thanks.

Update:

Rechecked the above issue on latest canary version 69.0.3497.0 on Mac(10.12.6, 10.13.1, 10.13.6, 10.14) and the content doesn't move down. Hence, the issue is found Fixed and adding respective labels.

Please refer the attached screencast.

Thank You..!!

Deleted: Fixed Video.mov 2.8 MB

	Fixed Video.mov 2.8 MB View Download

M69 branch #3497, branched at chromium revision #576753. No merge is needed.

Update:

Retested above issue and it is not fixed on build #70.0.3536.0 using Mac(10.12.6, 10.13.1, 10.13.6, 10.14) OS.

Thank you.

Deleted: Actual_Issue.mov 4.1 MB

	Actual_Issue.mov 4.1 MB View Download

Since this bug was fixed and verified earlier, and what is mentioned in #10 appears to be a regression, I filed a new bug https://bugs.chromium.org/p/chromium/issues/detail?id=879221 for that.