[css-scrollbars] What is "the background"

[frivoal]

In #9853, we resolved that:

transparent colors in the thumb are transparent towards the track, and transparent colors in the track are transparent towards the background […]

But what exactly is "the background"? As far as I can tell from testing, Firefox and Chrome do agree: this is the composited background of all elements that end up behind, including images, gradients… and if the backgrounds themselves are transparent, you include further backgrounds behind. It does not include the content of the scrolling element, but it does include the content of ancestors / things behind:

http://software.hixie.ch/utilities/js/live-dom-viewer/?saved=12876

Since implementations agree, I see no strong reason to propose anything else, but is there a pre-existing term to refer to this kind of background?

I'm guessing that it is effectively transparent towards everything, not just things that are technically background, and it is merely that the content of the scrolling element itself is clipped (to the padding box), so there's nothing to be transparent to.