position: sticky, and the parent that silently breaks it

It works in isolation and does nothing in the real page, roughly every time, and the reason is almost always an ancestor rather than the element itself.

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

/* any of these on an ancestor kills it: */
/*   overflow: hidden | auto | scroll   */
/*   the parent being shorter than the element                */
/*   a flex parent with align-items: stretch and no height    */

The overflow rule is the one that catches everyone, because overflow: hidden is applied casually to clear floats or hide a scrollbar and there is no warning that it disables sticky positioning three levels down. Sticky is also constrained to its parent, so a sidebar in a short container stops moving where that container ends — which reads as a bug and is the specification. Debugging it means walking up the tree in devtools rather than adjusting the element.