UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:61.0) Gecko/20100101 Firefox/61.0

Steps to reproduce the problem:
1. See an attached HTML file.

What is the expected behavior?
Left edge of the fixed element touches left edge of viewport.

What went wrong?
There is 17px gap (same as width of vertical scrollbar) between left edge of the fixed element and left edge of viewport.

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: 68.0.3402.0  Channel: canary
OS Version: 10.0
Flash Version: Shockwave Flash 29.0 r0

This issue happens when:

* The principal writing mode is `vertical-rl`,
* Viewport has vertical scrollbar, and
* CSS properties that may cause hardware acceleration, such as `animation` or `transform: translateZ(0)`, are applied to the absolutely positioned element itself or its descendants.
 

Deleted: 20180421-positioned-layer-in-vertical-layout.html 440 bytes

20180421-positioned-layer-in-vertical-layout.html 440 bytes View Download

Deleted: 20180421-positioned-layer-in-vertical-layout-on-chrome.png 2.4 KB

	20180421-positioned-layer-in-vertical-layout-on-chrome.png 2.4 KB View Download