fix(app): scroll store performance

packages/app/src/context/layout-scroll.test.ts

@@ -0,0 +1,73 @@
+import { describe, expect, test } from "bun:test"
+import { createRoot } from "solid-js"
+import { createStore } from "solid-js/store"
+import { makePersisted, type SyncStorage } from "@solid-primitives/storage"
+import { createScrollPersistence } from "./layout-scroll"
+
+describe("createScrollPersistence", () => {
+  test("debounces persisted scroll writes", async () => {
+    const key = "layout-scroll.test"
+    const data = new Map<string, string>()
+    const writes: string[] = []
+    const stats = { flushes: 0 }
+
+    const storage = {
+      getItem: (k: string) => data.get(k) ?? null,
+      setItem: (k: string, v: string) => {
+        data.set(k, v)
+        if (k === key) writes.push(v)
+      },
+      removeItem: (k: string) => {
+        data.delete(k)
+      },
+    } as SyncStorage
+
+    await new Promise<void>((resolve, reject) => {
+      createRoot((dispose) => {
+        const [raw, setRaw] = createStore({
+          sessionView: {} as Record<string, { scroll: Record<string, { x: number; y: number }> }>,
+        })
+
+        const [store, setStore] = makePersisted([raw, setRaw], { name: key, storage })
+
+        const scroll = createScrollPersistence({
+          debounceMs: 30,
+          getSnapshot: (sessionKey) => store.sessionView[sessionKey]?.scroll,
+          onFlush: (sessionKey, next) => {
+            stats.flushes += 1
+
+            const current = store.sessionView[sessionKey]
+            if (!current) {
+              setStore("sessionView", sessionKey, { scroll: next })
+              return
+            }
+            setStore("sessionView", sessionKey, "scroll", (prev) => ({ ...(prev ?? {}), ...next }))
+          },
+        })
+
+        const run = async () => {
+          await new Promise((r) => setTimeout(r, 0))
+          writes.length = 0
+
+          for (const i of Array.from({ length: 100 }, (_, n) => n)) {
+            scroll.setScroll("session", "review", { x: 0, y: i })
+          }
+
+          await new Promise((r) => setTimeout(r, 120))
+
+          expect(stats.flushes).toBeGreaterThanOrEqual(1)
+          expect(writes.length).toBeGreaterThanOrEqual(1)
+          expect(writes.length).toBeLessThanOrEqual(2)
+        }
+
+        void run()
+          .then(resolve)
+          .catch(reject)
+          .finally(() => {
+            scroll.dispose()
+            dispose()
+          })
+      })
+    })
+  })
+})