How to Display a Zero Height Line

[Psionic K]

Related dslide user issue:
https://github.com/positron-solutions/dslide/issues/15

If I use line height or line spacing to accomplish the slide-in
effect, text with any background color set have that background
stretched out.

To work around this, I was going to use a zero height line but could
not achieve the effect after a couple tries.

While I achieved vanishing line height in a fundamental mode buffer
using an actual newline with text properties, I could not reproduce
the effect in a fontified buffer, where I needed to use an overlay.  I
messed around a bit with 'before-string etc.

I'm aware this might be a display limitation and want to confirm
before labeling my user's issue accordingly.