React hydration and SSR: prevent mismatches without hiding content

In Next.js, a localStorage value can make server-rendered HTML differ from the first client render. The article compares the trade-offs: waiting until mount avoids the error but leaves the real content out of the SSR HTML; using the same initial value for both renders preserves that content, though a later update can cause flicker.

To synchronize a sidebar without that flash, an inline script applies its saved state to the DOM before hydration and passes it to React, which uses the same classes. The article also distinguishes an isolated theme control from a mobile backdrop handled with CSS.