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Both stable, but svh pins to the *smallest* viewport (toolbar visible), which never grows once mobile Safari auto-collapses its toolbar on the first scroll. The actual visible screen becomes taller than a box sized with svh, so content below it appears to snap into the newly-revealed space the moment the toolbar collapses - exactly the "text jumps down with the expanded area" behavior reported. lvh pins to the toolbar-collapsed size instead, matching the settled state scrolling actually lands on, so there's no gap to reveal. svh is the right choice for the opposite case (a fixed element that must never be hidden behind the toolbar), not a hero at the top of a scrolling page - wrong tool for this particular job.