This is a regression issue, broken in M-68 and providing the bisect using per-revision script:
Good Build : 68.0.3417.0 (Revision : 555218)
Bad Build : 68.0.3418.0 (Revision : 555651)

You are probably looking for a change made after 555255 (known good), but no later than 555256 (first known bad).

Change Log URL :
https://chromium.googlesource.com/chromium/src/+log/abf2ce0e99d7b92eeebed86018e186aec516d249..c0f34fbc93cc949bcf8c959b9d21b15cb83c8080

Suspect : https://chromium.googlesource.com/chromium/src/+/c0f34fbc93cc949bcf8c959b9d21b15cb83c8080

@dpapad : 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.

Note : 
1. Issue is also observed on Linux(14.04 LTS) OS but the same issue is not seen on Mac(10.12.6, 10.13.1, 10.13.5) OS.

Candidate fix at https://chromium-review.googlesource.com/c/chromium/src/+/1043268.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/20376bcfe574adb3f11e5984b39fffea90542d7f

commit 20376bcfe574adb3f11e5984b39fffea90542d7f
Author: dpapad <dpapad@chromium.org>
Date: Sat May 05 02:33:36 2018

Settings WebUI: Undo default box-shadow style for advanced toggle.

Bug:  839270 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I7af505de122b071846617c95dd20454214c49275
Reviewed-on: https://chromium-review.googlesource.com/1043268
Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556296}
[modify] https://crrev.com/20376bcfe574adb3f11e5984b39fffea90542d7f/chrome/browser/resources/settings/settings_menu/settings_menu.html

Update : 
Retested above issue in latest Canary build #68.0.3423.0 on Windows(7,8,8.1,10) & Linux(14.04 LTS) OS and the issue is fixed. Focus on 'Advanced' option appears proper in size. Kindly review an attached screen-cast.

Thank you..!

Deleted: Canary_Result.mp4 367 KB

	Canary_Result.mp4 367 KB View Download