Sometimes interior size of factory in preview doesn't match its original size

Reproducing: Quickly change the selection between two factories (the selected and the one "above" it).

That's a vanilla bug. It's already fixed for Factorio 0.15.10.

It's not fixed, because I'm using 0.15.9. And your link is on 0.15.7, instead of 0.15.10. =)

The bug report was created in 0.15.7, the fix will be released in 0.15.10. You'll notice that the thread is in the "Resolved for the next release" board.

I don't see a preview at all. As far as I know, I'm up to date, but I'm not sure what to do to see it. Just hover over the building with the mouse... either a placed one or in my inventory? I've been excited about this feature, but it doesn't seem to work.

There's a new technology you need to research first.