fix(app): throttle directory tree loading

packages/app/src/components/dialog-select-directory-v2.tsx

@@ -19,6 +19,7 @@ import {
   pickerMode,
   preloadTreeDirectories,
   cleanPickerInput,
+  createPriorityTaskQueue,
   createDirectorySearch,
   currentPickerSuggestions,
   displayPickerPath,
@@ -55,6 +56,7 @@ export function DialogSelectDirectoryV2(props: DialogSelectDirectoryV2Props) {
   const [error, setError] = createSignal(false)
   const [rootValid, setRootValid] = createSignal(false)
   const listings = new Map<string, Promise<Array<{ name: string; type: "file" | "directory" }> | undefined>>()
+  const loads = createPriorityTaskQueue<Array<{ name: string; type: "file" | "directory" }> | undefined>(3)
   const advanced = new Set<string>()
   let tree: FileTree | undefined
   let container: HTMLDivElement | undefined
@@ -102,16 +104,21 @@ export function DialogSelectDirectoryV2(props: DialogSelectDirectoryV2Props) {
   })
   const currentSuggestions = createMemo(() => currentPickerSuggestions(suggestions(), input()))
 
-  async function load(path: string, generation: number, preload = true) {
+  async function load(path: string, generation: number, eager = false) {
     const key = path.replace(/\/+$/, "")
     setError(false)
     const absolute = absoluteTreePath(root(), key)
+    const existing = listings.get(key)
+    if (existing && !eager) loads.promote(`${generation}:${key}`)
     const request =
-      listings.get(key) ??
-      sdk.client.file
-        .list({ directory: absolute, path: "" })
-        .then((result) => result.data ?? [])
-        .catch(() => undefined)
+      existing ??
+      loads.schedule(`${generation}:${key}`, eager ? "background" : "user", () => {
+        if (!activeTreeNavigation(generation, navigation)) return Promise.resolve(undefined)
+        return sdk.client.file
+          .list({ directory: absolute, path: "" })
+          .then((result) => result.data ?? [])
+          .catch(() => undefined)
+      })
     listings.set(key, request)
     const nodes = await request
     if (!activeTreeNavigation(generation, navigation)) return false
@@ -121,8 +128,8 @@ export function DialogSelectDirectoryV2(props: DialogSelectDirectoryV2Props) {
       return false
     }
     tree?.batch(policy.entries(key, nodes).map((item) => ({ type: "add", path: item })))
-    if (preload && advanceTreePreload(advanced, key)) {
-      void Promise.all(preloadTreeDirectories(key, nodes).map((directory) => load(directory, generation, false)))
+    if (!eager && advanceTreePreload(advanced, key)) {
+      for (const directory of preloadTreeDirectories(key, nodes)) void load(directory, generation, true)
     }
     return true
   }

packages/app/src/components/directory-picker-domain.test.ts

@@ -15,6 +15,7 @@ import {
   treePathWithin,
   currentPickerSuggestions,
   createDirectorySearch,
+  createPriorityTaskQueue,
   displayPickerPath,
   pickerParent,
   pickerRoot,
@@ -168,6 +169,41 @@ test("advances preloading once for every expanded directory", () => {
   expect(advanceTreePreload(advanced, "repos/")).toBeTrue()
 })
 
+test("limits background tasks and prioritizes newly requested work", async () => {
+  const queue = createPriorityTaskQueue<void>(2)
+  const first = Promise.withResolvers<void>()
+  const second = Promise.withResolvers<void>()
+  const started: string[] = []
+  let active = 0
+  let maximum = 0
+  const task = (name: string, blocker?: Promise<void>) => async () => {
+    started.push(name)
+    active++
+    maximum = Math.max(maximum, active)
+    await blocker
+    active--
+  }
+
+  const running = [
+    queue.schedule("first", "background", task("first", first.promise)),
+    queue.schedule("second", "background", task("second", second.promise)),
+    queue.schedule("preload", "background", task("preload")),
+    queue.schedule("opened", "user", task("opened")),
+  ]
+  await Promise.resolve()
+  expect(started).toEqual(["first", "second"])
+
+  first.resolve()
+  await running[0]
+  await Promise.resolve()
+  expect(started).toEqual(["first", "second", "opened"])
+
+  second.resolve()
+  await Promise.all(running)
+  expect(started).toEqual(["first", "second", "opened", "preload"])
+  expect(maximum).toBe(2)
+})
+
 test("clamps bridged tree wheel scrolling", () => {
   expect(nextTreeScrollTop(100, 40, 500, 200)).toBe(140)
   expect(nextTreeScrollTop(10, -40, 500, 200)).toBe(0)

packages/app/src/components/directory-picker-domain.ts

@@ -138,6 +138,79 @@ export function activeTreeNavigation(request: number, current: number) {
   return request === current
 }
 
+export function createPriorityTaskQueue<T>(concurrency: number) {
+  type Job = {
+    key: string
+    priority: "user" | "background"
+    promise: Promise<T>
+    run: () => void
+  }
+
+  const jobs = new Map<string, Job>()
+  const user: Job[] = []
+  const background: Job[] = []
+  let active = 0
+
+  const drain = () => {
+    while (active < concurrency) {
+      const job = user.pop() ?? background.shift()
+      if (!job) return
+      active++
+      job.run()
+    }
+  }
+
+  const schedule = (key: string, priority: Job["priority"], task: () => Promise<T>) => {
+    const existing = jobs.get(key)
+    if (existing) {
+      if (priority === "user") promote(key)
+      return existing.promise
+    }
+
+    const deferred = Promise.withResolvers<T>()
+    const job: Job = {
+      key,
+      priority,
+      promise: deferred.promise,
+      run: () => {
+        const complete = () => {
+          active--
+          jobs.delete(key)
+          drain()
+        }
+        Promise.resolve()
+          .then(task)
+          .then(
+            (value) => {
+              complete()
+              deferred.resolve(value)
+            },
+            (error) => {
+              complete()
+              deferred.reject(error)
+            },
+          )
+      },
+    }
+    jobs.set(key, job)
+    ;(priority === "user" ? user : background).push(job)
+    drain()
+    return job.promise
+  }
+
+  const promote = (key: string) => {
+    const job = jobs.get(key)
+    if (!job || job.priority === "user") return
+    const index = background.indexOf(job)
+    if (index === -1) return
+    background.splice(index, 1)
+    job.priority = "user"
+    user.push(job)
+  }
+
+  return { schedule, promote }
+}
+
 export function nextTreeScrollTop(current: number, delta: number, scrollHeight: number, clientHeight: number) {
   return Math.min(Math.max(0, scrollHeight - clientHeight), Math.max(0, current + delta))
 }