OS: Linux (didn't try on others)

What steps will reproduce the problem?
(1)enable chrome://flags/#overlay-scrollbars
(2)Go to bokan.ca/iframe.html
(3)Open dev tools, source tab

What is the expected output?
scrollbars must be idle (narrow and light), and while scrolling they shouldn't disappear.

What do you see instead?
They are dark and thick in the beginning, and disappear while scrolling.

I suspect that it's a new regression, because I couldn't reproduce it in
53.0.2785.116. 



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