You add sticky positioning to a sidebar, scroll down, and watch it disappear as before. Increasing its z-index changes nothing. The first useful question is not how to make the sidebar stronger, but which box its movement belongs to.
MDN’s positioning reference explains that sticky positioning relates to the nearest ancestor with a scrolling mechanism. An ancestor using overflow hidden, auto or scroll can establish that relationship even when it is not the box you expected to scroll. Sticky movement also remains constrained by its containing block.
Make the smallest version of the problem
In a local test page, create a long article beside a short sidebar. Give the sidebar position: sticky and a top inset such as 1rem. The value is an example for the test, not a universal layout prescription. Without a non-auto inset on the relevant axis, there is no useful threshold for that axis.
Scroll the page and note when the sidebar begins and stops moving. Keep this working example beside the real page rather than copying random declarations into production.
Inspect the route from sidebar to page
- Read the computed overflow values on each ancestor, including wrappers added by a component.
- Check whether the sidebar has room to move within its containing block.
- Check whether grid or flex sizing stretches the sidebar so that it occupies all available height.
- Test the actual element that scrolls, which may be a panel inside the page.
Change one condition in developer tools, record the result, then restore it before testing another. Removing overflow globally may reveal the cause while also breaking clipping elsewhere; that is a diagnostic observation, not automatically the final fix.
Finish with a smaller screen and enlarged text. A tall sidebar can be awkward even when its CSS works exactly as specified. Let it return to normal flow where that makes navigation easier. The successful outcome is accessible content and predictable movement, not a sticky effect at every viewport size.