A Dynamically-Sized Sticky Sidebar with HTML and CSS

November 20th, 2020

Creating page content that sticks to the viewport as you scroll, something like a jump-to-anchor menu or section headings, has never been easier. Throw a position: sticky into your CSS ruleset, set the directional offset (e.g. top: 0) and you’re ready to impress your teammates with minimal effort.

But sticky positioning can get a bit tricky, particularly when it comes to height and the dangerous situation of hiding content in a position that can’t be scrolled to. Let me set the stage and show you the problem and how I fixed it.

I recently worked on a desktop layout that we’re all familiar with: the main content area with a sidebar next to it. This particular sidebar contains action items and filters that are pertinent to the main content. As the page section is scrolled, this component remains fixed to the viewport and contextually accessible.

The layout styling was as easy to implement as I had mentioned earlier. But there was a catch: The height of the component would vary based on its content. I could have capped it with a max-height and set overflow-y: auto to make the component content scrollable. This worked well on my laptop screen and my typical viewport height, but in a smaller viewport with less vertical real estate, the sidebar’s height would exceed the viewport.

When the sticky sidebar height is larger than the viewport, some of its content becomes inaccessible until reaching the bottom of the container, when the element is no longer sticky.

That’s where things got tricky.

Thinking through solutions

I initially considered reaching for a media query. Perhaps I could use a media query to remove the sticky positioning and have the component sit relative to the top of the sidebar container. This would grant access to the entirety of its content. Otherwise, when scrolling the page, the sticky component’s content is cut off at the bottom of the viewport until I reach the end of its parent section.

Then I remembered that the height of the sticky component is dynamic.

What magic value could I use for my media query that would handle such a thing? Perhaps instead I could write a JavaScript function to check if the component flows beyond the viewport boundaries on page load? Then I could update the component’s height…

That was a possibility.

But what if the user resizes their window? Should I use that same function in a resize event handler? That doesn’t feel right. There must be a better way to build this.

Turns out there was and it involved some CSS trickery to get the job done!

Setting up the page section

I started with a flex display on the main element. A flex-basis value was set to the sidebar for a fixed desktop width. Then the article element filled the rest of the available horizontal viewport space.

If you’re curious about how I got the two containers to stack for smaller viewports without a media query, check out The Flexbox Holy Albatross trick.

I added align-self: start to the sidebar so its height wouldn’t stretch with the main article (stretch  is the default value). This gave my positioning properties the ability to cast their magic:

.sidebar {
  --offset: var(--space);
  /* ... */
  position: sticky;
  top: var(--offset);

Check that out! With these two CSS properties, the sidebar element sticks to the top of the viewport with an offset to give it some breathing room. Notice that the top value is set to a scoped CSS custom property. The --offset variable can now be reused on any child element inside the sidebar. This will come in handy later when setting the sticky sidebar component’s maximum height.

You can find a list of global CSS variable declarations in the CodePen demo, including the --space variable used for the offset value in the :root ruleset.

The sticky sidebar

Keep in mind that the component itself is not what is sticky; it’s the sidebar itself. General layout and positioning should typically be handled by the parent. This gives the component more flexibility and makes it more modular to use in other areas of the application.

Let’s dive into the anatomy of this component. In the demo, I’ve removed the decorative properties below to focus on the layout styles:

.component {
  display: grid;
  grid-template-rows: auto 1fr auto;

.component .content {
  max-height: 500px;
  overflow-y: auto;
  • This component uses CSS Grid and the pancake stack idea from 1-Line Layouts to configure the rows of this template. Both the header and footer (auto) adjust to the height of their children while the content (1fr, or one fraction unit) fills up the rest of the open vertical space.
  • A  max-height on the content limits the component’s growth on larger screen sizes. This is unnecessary if it’s preferred that the component stretch to fill the viewport height.
  • overflow-y: auto allows the content to be scrolled when necessary.

When the component is being used in the sidebar, a max-height is needed so that it doesn’t exceed the viewport height. The --offset previously scoped to the .sidebar class is doubled to create a margin on the bottom of the element that matches the top offset of the sticky sidebar:

.sidebar .component {
  max-height: calc(100vh - var(--offset) * 2);

That wraps up the assembly of this sticky sidebar component! After some decorative styles were applied, this prototype became ready for testing and review. Give it a try! Open up the demo in CodePen and click on the grid items to add them to the sidebar. Resize your browser window to see how the component flexes with the viewport while staying in view as you scroll the main content section.

This layout may work well on a desktop browser but isn’t entirely ideal for smaller devices or viewport widths. However, the code here provides a solid foundation that makes it easy to add improvements to the UI.

One simple idea: A button could be affixed to the viewport window that, when clicked, jumps the page down to the sidebar content. Another idea: The sidebar could be hidden off-screen and a toggle button could slide it in from the left or right. Iteration and user testing will help drive this experience in the right direction.

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