Slideshow Block: resizing / rendering issues

#### Steps to reproduce the issue

1. Use a Slideshow block
2. Look at it on the front end.

Video: https://d.pr/v/DiY5FP

Go to https://css-tricks.com/a-complete-guide-to-dark-mode-on-the-web/ and scroll down like 75% of the way, past the "Dark Mode in the Wild" header.

#### What I expected

The Slideshow not to go crazy with re-renders.

#### What happened instead

The Slideshow went crazy with the re-renders. But also some calculation ultimately makes it jiggle between 1px of height.

Start by reproducing the Slideshow block on the front end and review its rendering while scrolling the linked CSS-Tricks page to the reported section. Compare the resize calculations and render behavior against the video; done means the slideshow no longer enters repeated re-renders or jiggles between 1px of height.

Written by the indexing model from the issue text.