Chrome Version: 51.0.2704.63 (Official Build)2989ec7055a268f05554172b2207a34c0c842b8d-refs/branch-heads/2704@{#656} 32/64-bit.
OS: Windows (7, 8, 10), Mac (10.10.5)(10.11.4), Linux (ubuntu 14.04 LTS).

What steps will reproduce the problem?
1. Launch chrome, open 'DevTools' window and click on 'Toggle device toolbar' button.
2. Select any device from device drop-down list and then select 'Show rulers' from 'More options' menu list.
3. Now resize the devtools window towards L.H.S. (extreme left) an observe the vertical ruler.

Vertical ruler gets hide on resizing the devtools window.
Vertical ruler should seen properly on resizing the devtools window.

Note: When 'Responsive' mode is selected from device drop-down list, Vertical ruler is seen properly and doesn't get hide on resizing the devtools window.

This is a regression issue, broken in M-49 series, below is bisect info.
Good build: 49.0.2615.0 
Bad build:  49.0.2616.0

Narrow bisect:
https://chromium.googlesource.com/chromium/src/+log/c6d86ea8b31ab2c684cd98f467d1e11bbb303bcf..c7246ee039f232e3ff2eb1edb10797b47ef8b9c9?pretty=fuller&n=100 

Suspecting: r368163

Kindly review the attached screen-cast for reference.
 

Deleted: Actual behavior.mp4 2.1 MB

Actual behavior.mp4 2.1 MB Download