Generators.dark(*element*)

[Fil]

We add a CSS transition to the body’s color style and listen to transitionstart. (Trick found in https://github.com/bramus/style-observer.)

This enables us to listen when the actual color starts transitioning, and update dark based on the computed value of color-scheme.

Note that this works with any element within the body, so:

1. if we want to ensure that the user has not added their own transition definitions to the body, we could use another element — or even create one on purpose.

2. we can listen to dark mode changes inside a page, say if someone has a block that needs to be in a mode that is toggled separately from the main page, maybe for demonstration purposes. I don't know if that is useful enough in practice, but it doesn't take more code to support this, and it's more consistent with Generators.width(*element*). I've added such an example in the documentation—for testing purposes (I don't plan to ship this example in any case, but it could be documented in pangea as a side show).

If we think the transition might break or not work in all cases, we could keep listening to the user preference (media).

Here is a standalone (user land) version:

---
_theme: light
---

# dark transitions

```js
display(dark);
```

```js echo
const dark = Mutable(null);
const setDark = (value) => dark.value = value;
```

```js echo
const probe = document.body;
probe.style.setProperty("transition-property", "color");
probe.style.setProperty("transition-duration", "0.001s");
const update = () => setDark(getComputedStyle(probe)["color-scheme"]);
update();
probe.addEventListener("transitionstart", update);
invalidation.then(() => probe.removeEventListener("transitionstart", update));
```

closes #1339

[mbostock]

This looks promising. The other thing that was hard about fixing #1339 is that even if we know that we’ve changed the stylesheets, there’s no event (as far as I can tell — load doesn’t work) that lets you listen for when the changed stylesheets have taken effect, at least other than this transitionstart technique that you’ve found. I’m wary of setting a transition on the body though. And technically this only works if color changes at the same time as color-scheme, which isn’t guaranteed. And since this only affects preview in practice I’m not that motivated… 😬