UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:64.0) Gecko/20100101 Firefox/64.0

Steps to reproduce the problem:
1. In Windows settings, set scaling to a factor that is greater than 100%, e.g. 125%
2. Navigate to a webpage that uses a dark theme (e.g. imgur.com). This is not needed to reproduce the issue, but makes the bug much easier to see.
3. Open DevTools and dock to side (doesn't matter which side)
4. Open the device toolbar
5. Activate "Show device frame" in the toolbar overflow menu, if not already activated. This is also not needed to reproduce the issue, but helps with identifying the bug.
6. Select "iPhone 5/SE", "iPhone 6/7/8", or any other listed device in the devices menu for which Chrome draws a physical device frame
7. Adjust the width of the DevTools sidebar until a white border can be seen on either side of the displayed webpage

What is the expected behavior?
There should be no visual artifacts around the webpage.

What went wrong?
At certain widths of the DevTools sidebar, a white border can be seen to the left of the webpage. This is most clearly discernible on dark webpages with physical device frames drawn, but appears regardless of either condition.

Did this work before? N/A 

Chrome version: Version 69.0.3497.100 (Official Build) (64-bit)  Channel: stable
OS Version: 10.0
Flash Version:
 

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