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## Summary - let Virtua own the initial visible timeline range instead of passing every loaded row to `keepMounted` - populate the existing bounded retention window after the virtualizer reports its first settled viewport - cover a 10,000-row timeline to prevent an all-history initial mount regression ## Why `useTimelineRetention` initialized its retained-key set with every loaded timeline key. Those indices were passed to Virtua's `keepMounted`, effectively defeating virtualization during initial channel positioning until `onScrollEnd` pruned the set. On a large real channel this grew WebContent into multiple gigabytes and blocked the renderer main thread for 20+ seconds while WebKit laid out and painted the retained rows. Starting with no retained rows restores Virtua's visible-range mount; the existing reader-neighborhood and visual-tail retention is populated once the viewport is measured. ## Validation - `node --import ./test-loader.mjs --experimental-strip-types --test src/features/messages/ui/useTimelineRetention.test.mjs` - pre-push hook at `8e86a189de7e9a8f2cb119396c8f912ed9dacd6e`: branch-skew, desktop-check, desktop-typecheck, and all 4,671 desktop tests passed - manual ablation against PR #5599 on the affected profile: catastrophic channel-switch stalls disappeared ## Authorship disclosure Carl implemented and is posting this change on Wes's behalf. --------- Signed-off-by: Wes <wesbillman@users.noreply.github.com> Co-authored-by: Carl <c7ebe626f000404285d3686e1dc74cc07cc60a9754a150041ba132e14bd3e2ec@buzz.block.builderlab.xyz>