Description

This... makes me uncomfortable.

It uses the css content property to replace the contents of cell elements we want to hide with a no-op (which must be an image, as it's the only supported value for this value on non-pseudo elements at the moment). This removes the need to target the child element(s) with display: none, and thus works correctly when the content we want to hide is or contains a text node which cannot be targeted by CSS. (In the case of the linked issue, this is the text that makes offscreen posts searchable via control-F.)

We cannot simply display: none the target elements themselves, as we need them to continue to be rendered and have bounding boxes so that observers targeting them continue to work.

Not entirely sure where the height comes from if you remove height: 0; a linear gradient has no intrinsic height. I guess there's a default height?

Resolves #1543.

Using a gradient as an element content replacement supposedly requires Firefox 113, per MDN/caniuse. Show Originals smoke tested in Firefox 126, Chrome 126, and desktop Safari 17.4.1 (there's some jank when switching Show Originals on or off and then scrolling into a previously loaded area, but I believe that's the case with or without this PR and is due to Virtual Scroller Things™).

Testing steps