Spoiler box scrollbar problem

[pepperjack]

On occasion, I'm seeing spoiler boxes display grayed-out vertical scrollbars. They appear to have no function or purpose, and they occupy horizontal real estate.

That by itself would be irksome, but tolerable. It becomes less tolerable when the scrollbar decides to rapidly appear and disappear, causing all the content from the top of the spoiler box on down to jiggle vertically.

I took a couple screenshots and made a GIF to show what it looks like. This is at about one-sixth speed (GIPHY didn't want to let me make it faster):

Spoiler: From the NSFW board, but probably SFW, technically

I didn't scroll between shots; that's how the content moves on the screen.

I'm using Blackened Blue and browsing in Chrome. The spoiler I'm using as an example is located in this post, but it's not the first time I've encountered the phenomenon.

 

[alethiophile]

This is probably a browser issue at root. I've had it before on sites I try to write.

There's probably a way to fix it, but it would require diving into XF internals and/or themes. Does the issue still happen in other themes?