How can I make a sticky element stay within its scrolling section?

Set position: sticky on the sidebar and a non-auto inset on the axis where it should stick. For vertical sticking, that could be top: 1rem. Sticky positioning is offset relative to its nearest ancestor with a scrolling mechanism and its containing block. An ancestor with overflow: hidden, scroll, auto, or overlay can establish that scrolling mechanism, even if it is not the element you expect to be scrolling. [S1]

Here is a small example. It assumes the page scrolls normally and that .section is the containing block you intend to bound the sidebar:

.section {
  display: grid;
  grid-template-columns: minmax(0, 1fr) 14rem;
  gap: 2rem;
}

.sidebar {
  position: sticky;
  top: 1rem;
}

<section class="section">
  <main><!-- Section content goes here. --></main>
  <aside class="sidebar"><!-- Sidebar content goes here. --></aside>
</section>

The CSS reference describes the relationship between sticky positioning, the scrolling ancestor, and the containing block; this layout is an example, not a guarantee for every surrounding stylesheet. [S1]

First check that the element has position: sticky and that the axis you care about has a non-auto inset. For vertical sticking, inspect top. Without a non-auto inset on that axis, sticky behaves like relative positioning there. [S1]

Next, inspect the ancestors, starting with the nearest one. Look for overflow: hidden, scroll, auto, or overlay: an ancestor with one of these values can establish the scrolling mechanism sticky uses, even if it is not the element you consider the active scroller. [S1]

As a debugging test, temporarily change an ancestor’s overflow rule and scroll again. If the behavior changes, that ancestor is relevant to the setup. Treat this as a test, not an automatic instruction to remove the rule: consider what the rest of your layout requires before making a permanent change.

Check which element is the sidebar’s containing block and whether it is the section you want to use as the boundary. Sticky positioning is relative to its containing block as well as its nearest ancestor with a scrolling mechanism. [S1] Keeping the sidebar in the section’s markup is a useful starting point, but inspect the actual ancestor relationships and overflow rules rather than assuming the markup alone determines the result.

For example, compare your structure with this sketch:

<section class="content-section">
  <div class="article-copy">Section content</div>
  <aside class="section-sidebar">Sidebar content</aside>
</section>

Trace upward from .section-sidebar, noting which ancestors establish the relevant scrolling mechanism and which element is its containing block. Then check behavior near the section’s beginning and end. The reference describes these positioning relationships, but it does not prescribe a particular layout for every page. [S1]

Try these checks one at a time:

1. Confirm the sidebar has position: sticky.
2. Set a non-auto inset on the desired axis; for vertical sticking, try top: 1rem. [S1]
3. Inspect ancestors for overflow: hidden, scroll, auto, or overlay, which can establish the scrolling mechanism sticky responds to. [S1]
4. Identify the sidebar’s containing block and check whether it is the intended section. [S1]
5. Scroll through the section and change one relevant rule at a time so you can see which change affects the behavior.

For example, if the sidebar does not stick while the page scrolls, check for a missing top value first. If it already has one, trace its ancestors and inspect their overflow rules. This order focuses on the positioning relationships described by the reference; it cannot identify the cause in a particular page without examining that page’s styles. [S1]