We've all been there. You've got an element you want to be able to collapse and expand smoothly using CSS transitions, but its expanded size needs to be content-dependent. You've set
transition: height 0.2s ease-out. You've created a
collapsed CSS class that applies
height: 0. You try it out, and... the height doesn't transition. It snaps between the two sizes as if
transition had never been set. After some fiddling, you figure out that this problem only happens when the height starts out or ends up as
auto. Percentages, pixel values, any absolute units work as expected. But all of those require hard coding a specific height beforehand, rather than allowing it to naturally result from the size of the element content.
If you were hoping I had a magical, complete solution to this problem, I'm sorry to disappoint you. There's no one solution that achieves the desired effect without downsides. There are, however, multiple workarounds that each come with a different set of advantages and disadvantages, and in most use cases at least one of them will get the job done in an acceptable manner. I'll outline the major ones, and list out their ups and downs so you can hopefully pick the best one for your situation.
If you web search this problem, the
max-height approach will probably be mentioned in all of the first five to ten results. It's actually pretty unideal, but I thought it was worth including here for the sake of comparison.
It works like this: CSS values can only be transitioned to and from fixed unit values. But imagine we have an element whose
height is set to
auto, but whose
max-height is set to a fixed value; say,
1000px. We can't transition
height, but we can transition
max-height, since it has an explicit value. At any given moment, the actual height of the element will be the maximum of the
height and the
max-height. So as long as
max-height's value is greater than what
auto comes out to, we can just transition
max-height and achieve a version of the desired effect.
Implementation works like this: we set a
transition for the element's
transform property, then toggle between
transform: scaleY(1) and
transform: scaleY(0). These mean, respectively, "render this element at the same scale (on the y axis) that it starts out at" and "render this element at a scale of 0 (on the y axis)". Transitioning between these two states will neatly "squish" the element to and from its natural, content-based size.
Managing a CSS transition in CSS would be ideal, but as we're learning, sometimes it just isn't entirely possible.
The basic strategy is to manually do what the browser refuses to: calculate the full size of the element's contents, then CSS transition the element to that explicit pixel size.