Transitioning to Auto Top

I do know that is one thing Chris has needed without end, so it’s no shock he’s already obtained a incredible write-up only a day after the information broke. In truth, I first realized about it from his put up and was unable to dredge up any type of announcement. So, I assumed I’d jot some notes down as a result of it seems like a big improvement.

The information: transitioning to auto is now a factor! Nicely, it’s going to be a factor. Chrome Canary lately shipped help for it and that’s the one place you’ll discover it for now. And even then, we simply don’t know if the Chrome Canary implementation will discover its technique to the syntax when the function turns into official.

The issue

Right here’s the state of affairs. You could have a component. You’ve marked it up, plopped in contents, and utilized a bunch of kinds to it. Are you aware how tall it’s? After all not! Positive, we will ask JavaScript to guage the ingredient for us, however so far as CSS is anxious, the ingredient’s computed dimensions are unknown.

That makes it tough to, say, animate that ingredient from peak: 0 to peak: no matter. We have to know what “no matter” is and we will solely try this by setting a set peak on the ingredient. That manner, now we have numbers to transition from zero peak to that particular peak.

.panel {
peak: 0;
transition: peak 0.25s ease-in;

&.expanded {
peak: 300px;
}
}

However what occurs if that ingredient adjustments over time? Perhaps the font adjustments, we add padding, extra content material is inserted… something that adjustments the scale. We doubtless must replace that peak: 300px to no matter new mounted peak works greatest. This is the reason we regularly see JavaScript used to toggle issues that develop and contract in measurement, amongst different workarounds.

I say that is concerning the peak property, however we’re additionally speaking concerning the logical equal, block-size, in addition to width and inline-size. Or any course for that matter!

Transitioning to auto

That’s the purpose, proper? We have a tendency to succeed in for peak: auto when the peak dimension is unknown. From there, we let JavaScript calculate what that evaluates to and take issues from there.

The present Chrome implementation makes use of CSS calc() to do the heavy lifting. It acknowledges the auto key phrase and, true to its title, calculates that quantity. In different phrases, we will do that as a substitute of the fixed-height strategy:

.panel {
peak: 0;
transition: peak 0.25s ease-in;

&.expanded {
peak: calc(auto);
}
}

That’s actually it! After all, calc() is able to extra complicated expressions however the truth that we will provide it with only a imprecise key phrase about a component’s peak is darn spectacular. It’s what permits us to go from a set worth to the ingredient’s intrinsic measurement and again.

I needed to give it a attempt. I’m positive there are a ton of use instances right here, however I went with a floating button in a calendar element that signifies a sure variety of pending calendar invitations. Click on the button, and a panel expands above the calendar and divulges the invitations. Click on it once more and the panel goes again to the place it got here from. JavaScript is dealing with the clicking interplay, triggering a category change that transitions the peak in CSS.

A video in case you don’t really feel like opening Canary:

That is the related CSS:

.invite-panel {
peak: 0;
overflow-y: clip;
transition: peak 0.25s ease-in;
}

On click on, JavaScript units auto peak on the ingredient as an inline fashion to override the CSS:

<div class=”invite-panel” fashion=”peak: calc(auto)”>

The transition property in CSS lets the browser know that we plan on altering the peak property sooner or later, and to make it easy. And, as with every transition or animation, it’s a good suggestion to account for movement sensitivities by slowing down or eradicating the movement with prefers-reduced-motion.

What about show: none?

This is among the first questions that popped into my head once I learn Chris’s put up and he will get into that as effectively. Transitioning from a component from show: none to its intrinsic measurement is type of like going from peak: 0. It would appear to be a non-displayed ingredient has zero peak, but it surely truly does have a computed peak or auto until a selected peak is asserted on it.

So, there’s further work to do if we wish to transition from show: none in CSS. I’ll merely plop within the code Chris shared as a result of it properly demonstrates the important thing elements:

.ingredient {
/* laborious mode!! */
show: none;

transition: peak 0.2s ease-in-out;
transition-behavior: allow-discrete;

peak: 0;
@starting-style {
peak: 0;
}

&.open {
peak: calc-size(auto);
}
}

The ingredient begins with each show: none and peak: 0.

There’s an .open class that units the ingredient’s peak to calc(auto).

These are the 2 dots we have to join and we do it by first setting transition-behavior: allow-discrete on the ingredient. That is new to me, however the spec says that transition-behavior “specifies whether or not transitions shall be began or not for discrete properties.” And after we declare allow-discrete, “transitions shall be began for discrete properties in addition to interpolable properties.”

Nicely, DevTools confirmed us proper there that peak: auto is a discrete property! Discover the @starting-style declaration, although. In the event you’re unfamiliar with it, you’re not alone. The concept is that it lets us set a mode for a transition to “begin” with. And since our ingredient’s discrete peak is auto, we have to inform the transition to start out at peak: 0 as a substitute:

.ingredient {
/* and so on. */

@starting-style {
peak: 0;
}
}

Now, we will transfer from zero to auto since we’re sorta overriding the discrete peak with @starting-style. Fairly cool we will try this!

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