Below is manual regression range.

Good build: 65.0.3284.0
Bad build: 65.0.3285.0

You are probably looking for a change made after 521548 (known good), but no later than 521549 (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/65b472737b0c1100ee672b9b89c3d2439defb654..09f677bd1562c40825b936f2af6c4d2232a3d585

Suspect: https://chromium.googlesource.com/chromium/src/+/09f677bd1562c40825b936f2af6c4d2232a3d585

@aboxhall: Kindly help to reassign, if your changes are not related to this issue.

NOTE: This issue is also reproducible on Mac OS X(10.12.6,10.13.2) and Linux (14.04 LTS) OS.

Kindly review the attached screen-cast for reference.

Deleted: Actual behavior.mp4 1.6 MB

	Actual behavior.mp4 1.6 MB View Download

Deleted: Expected behavior.mp4 920 KB

	Expected behavior.mp4 920 KB View Download

Very plausible. Will investigate tomorrow. Feel free to roll that change back if this is causing issues.

Adding RB Label as this is a recent Regression. Please remove if not required.
Thank You.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e7898d1ac109b4081cba963ea6c4aab9704ec135

commit e7898d1ac109b4081cba963ea6c4aab9704ec135
Author: Alice Boxhall <aboxhall@chromium.org>
Date: Wed Dec 06 00:57:29 2017

[Devtools] Fix two style issues caused by rearranging the Color Picker

Bug:  791906 , 791902 
Change-Id: I4a4ccb4afd70b4888a14455f56eea3ba739add77
Reviewed-on: https://chromium-review.googlesource.com/809810
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Alice Boxhall <aboxhall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521927}
[modify] https://crrev.com/e7898d1ac109b4081cba963ea6c4aab9704ec135/third_party/WebKit/Source/devtools/front_end/color_picker/Contrast.js
[modify] https://crrev.com/e7898d1ac109b4081cba963ea6c4aab9704ec135/third_party/WebKit/Source/devtools/front_end/color_picker/Spectrum.js
[modify] https://crrev.com/e7898d1ac109b4081cba963ea6c4aab9704ec135/third_party/WebKit/Source/devtools/front_end/color_picker/spectrum.css

Update:-
 Tested this issue on Windows (7,8,10) & Mac OS X(10.12.6,10.13.2) machines using latest Chrome canary build# 65.0.3286.0 fix is working as expected i.e. Alternate shades window seen properly. hence adding TE Verified labels.

Please find the attached screen-cast for reference.

Thanks..!

Deleted: LatestCanary behavior.mp4 1.0 MB

	LatestCanary behavior.mp4 1.0 MB View Download

Update:-
 Tested this issue on Linux (14.04 LTS) machine using latest Chrome canary build# 65.0.3287.0 fix is working as expected i.e. Alternate shades window seen properly.

Thanks.