URL: http://devdocs.io/settings

Browser / Version: Firefox 60.0
Operating System: Windows 10
Tested Another Browser: Yes

Problem type: Something else
Description: Scrolled area canvas blank repaint while scrolling
Steps to Reproduce:
Seems like a recent regression: been using Nightly and Devdocs before and cannot recall such apparent 'tearing'.
Observed such blank scrolled layer 'flashes' on other sites as well, but with very varying reproducibility. Mostly just brief flash of blankness occurs during (manual) scroll, but here at devdocs I've observer "persistent blankness until scrollbar was dropped".

Nightly 60.0a1 build 20180211220056

Screengrab if attachment fails (yes, it fails):
https://i.imgur.com/giiiN3N.gif

gfx.webrender.all: false
gfx.webrender.blob-images: 1
gfx.webrender.enabled: false
image.mem.shared: 2

gfx.webrender.all: false
gfx.webrender.blob-images: 1
gfx.webrender.enabled: false
image.mem.shared: 2

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