perf(board): reduce mobile reverse-scroll jank

docs/agent-runs/mobile-virtuoso-scroll-jank/feature-list.json

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+{
+  "task": "mobile-virtuoso-scroll-jank",
+  "last_updated": "2026-03-31",
+  "items": [
+    {
+      "id": "F001",
+      "priority": 1,
+      "status": "completed",
+      "description": "Reproduce, profile, and fix mobile-only scroll jank on Virtuoso-backed board feeds, starting with /all using the custom Node RPC.",
+      "verification": [
+        "./scripts/agent-init.sh --smoke",
+        "playwright-cli mobile verification on /all with the custom RPC configured",
+        "yarn build",
+        "yarn lint",
+        "yarn type-check",
+        "yarn doctor"
+      ],
+      "files": [
+        "src/components/comment-media/comment-media.tsx",
+        "src/views/board/board.tsx",
+        "src/views/board/__tests__/board.test.tsx",
+        "docs/agent-runs/mobile-virtuoso-scroll-jank/feature-list.json",
+        "docs/agent-runs/mobile-virtuoso-scroll-jank/progress.md"
+      ],
+      "notes": "Work stayed isolated in a dedicated worktree/branch. Profiling confirmed the mobile multiboard Virtuoso feed was underestimating row height and remounting tall rows too aggressively on reverse scroll. A second pass removed board-state snapshotting from the hot scroll path, added stronger reverse-direction overscan for mobile multiboards, and deduplicated GIF first-frame work in CommentMedia. Desktop comparison remained partially inconclusive because fresh desktop sessions sometimes failed to hydrate feed rows."
+    }
+  ]
+}

docs/agent-runs/mobile-virtuoso-scroll-jank/progress.md

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+# Progress Log
+
+Append one entry per session.
+
+## 2026-03-31 19:10
+
+- Item: F001
+- Summary: Created a dedicated fix worktree, loaded the profiling workflow, and installed dependencies in the fresh checkout after the required smoke bootstrap initially failed to resolve `playwright`.
+- Files: `docs/agent-runs/mobile-virtuoso-scroll-jank/feature-list.json`, `docs/agent-runs/mobile-virtuoso-scroll-jank/progress.md`
+- Verification: `./scripts/create-task-worktree.sh fix mobile-virtuoso-scroll-jank`, `corepack yarn install`
+- Blockers: `./scripts/agent-init.sh --smoke` failed before install because the fresh worktree had no local dependency install yet.
+- Next: Start a branch-scoped dev server, rerun the smoke flow against that URL, then profile the mobile /all feed with the custom RPC.
+
+## 2026-03-31 19:24
+
+- Item: F001
+- Summary: Profiled mobile `/all` with the custom RPC, confirmed Virtuoso row-height mismatch on mobile multiboards, then raised the mobile board-feed default item height and viewport buffer while making the board scroll-state listener passive.
+- Files: `src/views/board/board.tsx`, `src/views/board/__tests__/board.test.tsx`, `docs/agent-runs/mobile-virtuoso-scroll-jank/feature-list.json`, `docs/agent-runs/mobile-virtuoso-scroll-jank/progress.md`
+- Verification: `AGENT_APP_URL=http://127.0.0.1:1356 ./scripts/agent-init.sh --smoke`, `yarn test src/views/board/__tests__/board.test.tsx`, `yarn test`, `yarn build`, `yarn lint`, `yarn type-check`, `yarn doctor`, `playwright-cli` mobile `/all` load-and-scroll check, Playwright mobile `/all` profiling probe
+- Blockers: Fresh desktop comparator sessions intermittently failed to hydrate `/all` rows, so desktop scroll-jank comparison stayed inconclusive even though mobile evidence was sufficient to isolate the board-feed issue.
+- Next: Ready for user review on `codex/fix/mobile-virtuoso-scroll-jank`.
+
+## 2026-03-31 19:44
+
+- Item: F001
+- Summary: After the first pass still reproduced reverse-scroll stutter, shifted the board feed off the per-scroll `getState()` path, increased mobile multiboard reverse overscan, and removed duplicate GIF first-frame work inside `CommentMedia`.
+- Files: `src/views/board/board.tsx`, `src/views/board/__tests__/board.test.tsx`, `src/components/comment-media/comment-media.tsx`, `docs/agent-runs/mobile-virtuoso-scroll-jank/feature-list.json`, `docs/agent-runs/mobile-virtuoso-scroll-jank/progress.md`
+- Verification: `yarn test src/views/board/__tests__/board.test.tsx`, `playwright-cli` mobile `/all` repro with the custom RPC on `http://127.0.0.1:1356`, `yarn test`, `yarn type-check`, `yarn lint`, `yarn build`, `yarn doctor`
+- Blockers: Headless scripted scroll traces are noisy for long-task timing, so browser evidence is strongest on reduced remount churn and removal of duplicate GIF thumbnail work rather than a perfectly stable synthetic benchmark.
+- Next: Have the user retry the exact mobile `/all` reverse-scroll path on this branch and confirm whether the remaining hitch is gone or narrow any residual stutter further.