PDF app with a reading ruler?

I recently discovered an excellent feature in the Kindle app: a “reading ruler.” When enabled, the app displays a translucent coloured bar over the text at a certain position in the page. Screenshot:

You can change the colour, the height, and the style of the bar.

I find it speeds up my reading considerably, but I would never want to import most of my readings (PDFs) into the Kindle app. Is anyone aware of a PDF reader with such a feature? Ideally one that can open files in place (to avoid the import-export do-see-doh).

I’ve looked at Highlights, PDF Expert, PDF Viewer, and Flexcil, to no avail.

And yes, I realize I could probably use an actual physical reading ruler (gasp) to accomplish this, app-agnostic… I’m thinking about it!

Skim has this built in, on the Mac.

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Ah, that’s great, thanks! I already use Skim but hadn’t thought to check it.

(I’m hoping for iPad options, too… should’ve mentioned that.)