1 Setting up

A Scrollama document consists of “step elements”. As you scroll down/up the document, a step element may enter or exit an offset threshold, which you can imagine as a horizontal line placed at a certain height of the viewport.

For this document, the step elements are level-one sections. Every time a level-one section enters the offset threshold, a class is-active is added to it. When it exits the threshold, the class is removed. To better understand the offset threshold, you can turn on the debug option in rolldown::scrollama_setup() so you can see the horizontal line:

rolldown::scrollama_setup(
  list(step = '.level1', offset = .2, debug = TRUE)
)

Note that rolldown::scrollama_setup() should be actually called at the end of a document.

Below is the CSS applied to this document. Basically we added borders to level-one sections, and background colors to “active” sections.

.level1 {
  min-height: 400px;
  border: 1px solid;
  margin-bottom: 4em;
  padding: 1em 2em 2em;
}
.is-active {
  background-color: yellow;
}
body {
  margin-bottom: 80vh;
}

2 Text

Example text.

3 Plots

You may include any number of plots in a section.

par(mar = c(4, 4, 0.5, 0.1))
plot(pressure, type = "h", las = 1)

4 For experts

As mentioned in the beginning, you should call rolldown::scrollama_setup() at the end of a document. If you know JavaScript, you may have noticed that scrollama_setup() is a simple helper function to write out JavaScript like this:

(function() {
  var scroller = scrollama();
  scroller.setup({"step": ".level1", "offset": 0.2})
    .onStepEnter(res => {
      res.element.classList.add("is-active");
    })
    .onStepExit(res => {
      res.element.classList.remove("is-active");
    });
  window.addEventListener("resize", scroller.resize);
})();

You certainly do not have to rely on this R helper function, and can write JavaScript directly in an R Markdown document. For example, if you want to use the class name current instead of is-active, you may set up Scrollama with a js code chunk:

```{js, echo=FALSE}
(function() {
  var scroller = scrollama();
  scroller.setup({"step": ".level1", "offset": 0.2})
    .onStepEnter(res => {
      res.element.classList.add("current");
    })
    .onStepExit(res => {
      res.element.classList.remove("current");
    });
  window.addEventListener("resize", scroller.resize);
})();
```

For more information about Scrollama, please check out its documentation at https://github.com/russellgoldenberg/scrollama.