Hide Scrollbars During an Animation

CSS still cannot be animate to the automatic size.

 .dropdown {
  transition: 0.2s;
  height: 0;
}
.dropdown.open {
  /* The height will change, but will not be animated. */
  height: auto;
}

You can try JavaScript tricks. Brandon Smith not long ago outlined several techniques here. I've been thinking of this solution because it's very simple:

 .dropdown {
  transition: 0.2s;
  max-height: 0;
}
.dropdown.open {
  /* ? */
  max-height: 400px;
}

Now that we have this magic number of 400px, it's really not ideal. However, the fact that this method works and is so simple makes it extremely attractive that I have always used it in production environments.

But magical numbers are not the only problem. Another problem is the scrollbar.

When we set max-height: 0; we also need overflow: hidden; to make sure the dropdown menu is actually hidden when closed. When the dropdown menu opens, we should probably use overflow: auto; so that we can not accidentally cut off the content if the natural height of the dropdown menu exceeds the expanded max-height . Using overflow: auto; solves this problem, but introduces another problem: During expansion, our drop-down menu will always display scrollbars for at least part of the expansion time, even if they are not required for the final expansion height. This is embarrassing!

CSS tips come to rescue.

We can still use overflow: auto; in the expanded state - we just need to overwrite it during the animation. As we learned in the great CSS specificity battle, @keyframes has the amazing ability to cover anything while they are active. Let's use them not for animations, but for this hidden scrollbar function:

 .dropdown {
  max-height: 0;
  overflow: hidden;
  transition: max-height 1.2s ease-in-out;
}
.dropdown.open {
  overflow: auto;
  max-height: 400px;
  animation: hide-scroll 1.2s backwards;
}
@keyframes hide-scroll {
  from, to { overflow: hidden; }
}

That's all!

Try adjusting the height to a smaller value and see how you don't see the scrollbar during the animation, but only at the end if needed. This causes a little jitter when the scrollbar pops up, but in my case it's acceptable as it rarely happens. If you absolutely want to stop jitter, you may need to always apply the (custom) scrollbar to the drop-down menu and may adjust the style of the scrollbar during the animation as needed.

Thanks to Mr. Stephen Shaw of @keyframers for this trick. While I was working on something on CodePen, I pulled him to help me solve this problem. We decided to turn this trick into a CodePen channel video, showing the collaboration mode, and we used it to find the problem/solution:

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