Is there a stack that does this? I'm not sure what you call it!

RapidWeaver Classic
1

It might be called a slide over. One image behind another and the user slides a bar to reveal the other image. See this example:

2

There’s a few that do this. One I have used is from Doobox: https://www.doobox.co.uk/stacks_store/demos/compare.html

4

There’s also Image Compare from @willwood

https://stacks4stacks.com/imagecompare/

5

Not an answer but there are some nice reveal effects on this page as you scroll down.

6

Those from Doobox and Will Woodgate are very nice. Thanks for pointing me to them.

7

Yes, in fact the Compare stack of @Doobox is a good option, alternatively the ImageCompare of @willwood. I have them both, 2 good choices.