Chrome version :51.0.2670.1 51d7412f2dfd496654331ba621822cc5bbcbda7b-refs/branch-heads/2670@{#1} (32/64 bit)
OS : Windows(Win 7 aero enabled),Linux.

Steps : 
1. Launch Chrome and go to www.google.com.
2. Now open devtools and select 'Responsive mode' and drag the devtools window to complete LHS.
3. Now press 'F1' key from keyboard to open 'Preferences' and then hover the mouse to any drop down list and observe

Actual : Content under the 'Preference'(Sources,Profiler,Network...) moves to RHS on mouse hover
Expected : Content should not move on mouse hover

This is a regression issue broken in 'M-50' and below is the manual regression and narrow bisect info.
Good build : 50.0.2625.0
Bad build : 50.0.2626.0

Narrow Bisect :
https://chromium.googlesource.com/chromium/src/+log/4a2221f01c47e2dda94469440be49f66b16d4bbf..0b95bd78dc3ac8936f7dec1dce92ec1118d04f95?pretty=fuller&n=50

Suspecting r370177 from narrow bisect

@dgozman : Could you please help to reassign if your change is not the cause for this issue

Note : Issue is not seen on Mac OS.
 

Deleted: Actual_video.mp4 465 KB

Actual_video.mp4 465 KB Download

Deleted: Expected_video.mp4 483 KB

Expected_video.mp4 483 KB Download