This extension is a (jankily made) implementation of reveal-code-focus. I stripped somethings out of it, and then tried to make sure it would still work with the original line-highlight plugin.

See the demo.

quarto add reuning/codefocus

This will install the extension under the _extensions subdirectory. If you're using version control, you will want to check in this directory.

If you want to highlight and explain specific pieces of code as you go through it you'll include the explanations in fragments under the code. For example:

---
title: "Codefocus Example"
format:
  revealjs: default
revealjs-plugins:
  - codefocus
execute:
  echo: true
---

```{r}
vector <- c(1,2,3)
vector <- sample(vector)
sum(vector)
```

::: {.fragment .current-only data-code-focus="1"}
When this fragment is shown, the first line of code will have the `focus` class added to it and it gets highlighted.
:::

In addition, .current-only will make the fragment disappear when you move into other fragments.

Here is the source code for a minimal example: example.qmd and the Revealjs presentation

Thanks also to shafayetShafee.