perf(runtime): remove timer clamps from cooperative yields

Renderer paste and input loops can schedule more than a thousand zero-delay timer yields for a maximum-size payload. Chromium clamps nested timers to 4ms, adding seconds of idle wall time.

Use MessageChannel tasks in renderer runtimes and setImmediate in Node while retaining a timer fallback for tests and unsupported environments.

src/main/browser/browser-text-insertion.ts

@@ -1,4 +1,5 @@
 import { measureClipboardTextByteLength } from '../../shared/clipboard-text'
+import { yieldToEventLoop } from '../../shared/event-loop-yield'
 import type { CdpCommandSender } from './snapshot-engine'
 
 export const BROWSER_TEXT_INSERT_CHUNK_BYTES = 64 * 1024
@@ -71,7 +72,7 @@ export async function insertTextThroughCdp(
     // process responsive between bounded CDP payloads.
     chunk = chunks.next()
     if (options?.yieldBetweenChunks !== false && !chunk.done) {
-      await new Promise<void>((resolve) => setTimeout(resolve, 0))
+      await yieldToEventLoop()
     }
   }
 }

src/main/opencode-usage/scanner.ts

@@ -145,7 +145,7 @@ export async function getProcessedDatabaseInfo(
 }
 
 async function yieldToEventLoop(): Promise<void> {
-  await new Promise((resolve) => setTimeout(resolve, 0))
+  await new Promise((resolve) => setImmediate(resolve))
 }
 
 function getProjectJoin(db: Database.Database): string {

src/renderer/src/components/dictation/dictation-insertion-target.ts

@@ -1,3 +1,4 @@
+import { yieldToEventLoop } from '../../../../shared/event-loop-yield'
 import {
   TEXT_CONTROL_PASTE_CHUNK_MAX_BYTES,
   TEXT_CONTROL_PASTE_DIRECT_MAX_BYTES,
@@ -180,7 +181,3 @@ function getNextDictationChunkBoundary(text: string, startIndex: number, maxByte
 
   return index
 }
-
-function yieldToEventLoop(): Promise<void> {
-  return new Promise((resolve) => globalThis.setTimeout(resolve, 0))
-}

src/renderer/src/components/editor/monaco-large-text-paste.ts

@@ -1,4 +1,5 @@
 import type { editor } from 'monaco-editor'
+import { yieldToEventLoop } from '../../../../shared/event-loop-yield'
 import {
   measureTextControlPasteByteLength,
   measureTextControlPasteByteLengthWithYield
@@ -158,10 +159,6 @@ function setCollapsedSelection(monacoEditor: MonacoPasteEditor, position: Positi
   })
 }
 
-function yieldToEventLoop(): Promise<void> {
-  return new Promise((resolve) => globalThis.setTimeout(resolve, 0))
-}
-
 async function insertMonacoTextInChunks(
   monacoEditor: MonacoPasteEditor,
   text: string,

src/renderer/src/components/editor/rich-markdown-large-text-paste.ts

@@ -1,5 +1,6 @@
 import type { Editor } from '@tiptap/react'
 import { toast } from 'sonner'
+import { yieldToEventLoop } from '../../../../shared/event-loop-yield'
 import {
   measureTextControlPasteByteLength,
   measureTextControlPasteByteLengthWithYield
@@ -87,10 +88,6 @@ function getNextChunkBoundary(text: string, startIndex: number, maxBytes: number
   return index
 }
 
-function yieldToEventLoop(): Promise<void> {
-  return new Promise((resolve) => globalThis.setTimeout(resolve, 0))
-}
-
 function isEditorAvailable(
   editor: Editor,
   canContinue: RichMarkdownLargeTextPasteOptions['canContinue']

src/renderer/src/components/terminal-pane/pty-input-write-queue.ts

@@ -1,3 +1,4 @@
+import { yieldToEventLoop } from '../../../../shared/event-loop-yield'
 import {
   isTerminalInputTooLargeWithDeferredMeasurement,
   iterateTerminalInputChunks
@@ -28,16 +29,12 @@ export type PtyInputWriteQueueDeps = {
   yieldBetweenWrites?: () => Promise<void>
 }
 
-function defaultYieldBetweenWrites(): Promise<void> {
-  return new Promise((resolve) => setTimeout(resolve, 0))
-}
-
 function isCoalescibleText(text: string): boolean {
   return text.length <= TERMINAL_INPUT_COALESCE_MAX_CODE_UNITS
 }
 
 export function createPtyInputWriteQueue(deps: PtyInputWriteQueueDeps): PtyInputWriteQueue {
-  const yieldBetweenWrites = deps.yieldBetweenWrites ?? defaultYieldBetweenWrites
+  const yieldBetweenWrites = deps.yieldBetweenWrites ?? yieldToEventLoop
   let pending: PendingPtyInputWrite[] = []
   let drainPromise: Promise<void> | null = null
 

src/renderer/src/components/terminal-pane/pty-transport.ts

@@ -10,6 +10,7 @@ import {
   isTerminalInputTooLargeWithDeferredMeasurement,
   iterateTerminalInputChunks
 } from '../../../../shared/terminal-input'
+import { yieldToEventLoop } from '../../../../shared/event-loop-yield'
 import { isRuntimeOwnedSshTargetId } from '../../../../shared/execution-host'
 import {
   ptyDataHandlers,
@@ -622,10 +623,6 @@ export function createIpcPtyTransport(opts: IpcPtyTransportOptions = {}): PtyTra
     commit: clearAccumulatedState
   }
 
-  function yieldToInputWriteDrain(): Promise<void> {
-    return new Promise((resolve) => setTimeout(resolve, 0))
-  }
-
   async function writeAcceptedPtyInput(id: string, data: string): Promise<boolean> {
     try {
       const tooLarge = isTerminalInputTooLargeWithDeferredMeasurement(data)
@@ -644,7 +641,7 @@ export function createIpcPtyTransport(opts: IpcPtyTransportOptions = {}): PtyTra
         }
         chunk = chunks.next()
         if (!chunk.done) {
-          await yieldToInputWriteDrain()
+          await yieldToEventLoop()
         }
       }
       return true

src/renderer/src/components/terminal-pane/terminal-paste-executor.ts

@@ -1,3 +1,4 @@
+import { yieldToEventLoop } from '../../../../shared/event-loop-yield'
 import { BRACKETED_PASTE_END, BRACKETED_PASTE_START } from './terminal-bracketed-paste'
 import { iterateTerminalPastePlanChunks } from './terminal-paste-chunks'
 import { createRedactedPasteExecutionDiagnostic } from './terminal-paste-diagnostics'
@@ -30,7 +31,7 @@ export async function executeTerminalPastePlan(
     writePty,
     isTargetCurrent,
     canContinue,
-    yieldToEventLoop = defaultYieldToEventLoop,
+    yieldToEventLoop: yieldBetweenChunks = yieldToEventLoop,
     operationTimeoutMs = getTerminalPasteOperationTimeoutMs(plan),
     now = defaultNow
   }: ExecuteTerminalPastePlanArgs
@@ -101,7 +102,7 @@ export async function executeTerminalPastePlan(
     } else if (chunk === BRACKETED_PASTE_END) {
       bracketedPasteOpen = false
     }
-    await yieldToEventLoop()
+    await yieldBetweenChunks()
   }
   return finish('pasted', chunksWritten)
 }
@@ -140,7 +141,3 @@ function result(
 function defaultNow(): number {
   return globalThis.performance?.now?.() ?? Date.now()
 }
-
-function defaultYieldToEventLoop(): Promise<void> {
-  return new Promise((resolve) => setTimeout(resolve, 0))
-}

src/renderer/src/lib/agent-draft-paste-content.ts

@@ -1,3 +1,4 @@
+import { yieldToEventLoop } from '../../../shared/event-loop-yield'
 import type { GlobalSettings } from '../../../shared/types'
 import {
   BRACKETED_PASTE_END,
@@ -159,7 +160,7 @@ async function isSanitizedDraftPasteOverLimit(content: string, maxBytes: number)
       index += 1
     }
     if (index >= nextYieldAt) {
-      await yieldToAgentDraftPastePreflight()
+      await yieldToEventLoop()
       nextYieldAt = index + AGENT_DRAFT_PASTE_PREFLIGHT_YIELD_CODE_UNITS
     }
   }
@@ -187,10 +188,6 @@ function getUtf8ByteLengthForCodePoint(codePoint: number): number {
   return 4
 }
 
-function yieldToAgentDraftPastePreflight(): Promise<void> {
-  return new Promise((resolve) => globalThis.setTimeout(resolve, 0))
-}
-
 async function writeAgentDraftPtyInput(
   settings: Pick<GlobalSettings, 'activeRuntimeEnvironmentId'> | null | undefined,
   ptyId: string,

src/renderer/src/lib/paste-payload-metadata.ts

@@ -1,3 +1,5 @@
+import { yieldToEventLoop } from '../../../shared/event-loop-yield'
+
 export type PastePayloadMetadata = {
   byteLength: number
   exceededLimit: boolean
@@ -62,7 +64,7 @@ export async function measurePastePayloadMetadataWithYield(
     1,
     options.yieldAfterCodeUnits ?? PASTE_PAYLOAD_METADATA_YIELD_CODE_UNITS
   )
-  const yieldToEventLoop = options.yieldToEventLoop ?? defaultPastePayloadMetadataYield
+  const yieldBetweenBatches = options.yieldToEventLoop ?? yieldToEventLoop
   let nextYieldAt = yieldAfterCodeUnits
   let byteLength = 0
   let hasControlSequences = false
@@ -89,7 +91,7 @@ export async function measurePastePayloadMetadataWithYield(
       index += 1
     }
     if (index >= nextYieldAt) {
-      await yieldToEventLoop()
+      await yieldBetweenBatches()
       nextYieldAt = index + yieldAfterCodeUnits
     }
   }
@@ -149,7 +151,3 @@ function getUtf8ByteLengthForCodePoint(codePoint: number): number {
   }
   return 4
 }
-
-function defaultPastePayloadMetadataYield(): Promise<void> {
-  return new Promise((resolve) => setTimeout(resolve, 0))
-}

src/renderer/src/lib/primary-selection-paste.ts

@@ -1,3 +1,4 @@
+import { yieldToEventLoop } from '../../../shared/event-loop-yield'
 import { isPrimarySelectionTextControl } from './primary-selection-capture'
 import {
   TEXT_CONTROL_PASTE_CHUNK_MAX_BYTES,
@@ -157,10 +158,6 @@ function insertContentEditableChunk(target: HTMLElement, range: Range, text: str
   return range
 }
 
-function yieldToEventLoop(): Promise<void> {
-  return new Promise((resolve) => globalThis.setTimeout(resolve, 0))
-}
-
 async function pasteLargeTextIntoContentEditable(
   target: HTMLElement,
   text: string,

src/renderer/src/lib/text-control-paste.ts

@@ -1,3 +1,4 @@
+import { yieldToEventLoop } from '../../../shared/event-loop-yield'
 import {
   createTextControlCancelledResult,
   createTextControlPastedResult,
@@ -186,10 +187,6 @@ function getSelectionRange(target: HTMLInputElement | HTMLTextAreaElement): {
   }
 }
 
-function yieldToEventLoop(): Promise<void> {
-  return new Promise((resolve) => globalThis.setTimeout(resolve, 0))
-}
-
 function defaultNow(): number {
   return globalThis.performance?.now?.() ?? Date.now()
 }

src/shared/clipboard-text.ts

@@ -1,3 +1,5 @@
+import { yieldToEventLoop } from './event-loop-yield'
+
 export const CLIPBOARD_TEXT_READ_MAX_BYTES = 16 * 1024 * 1024
 export const CLIPBOARD_TEXT_WRITE_MAX_BYTES = 16 * 1024 * 1024
 export const CLIPBOARD_TEXT_TOO_LARGE_ERROR = 'Clipboard text is too large for this paste target.'
@@ -53,7 +55,7 @@ export async function measureClipboardTextByteLengthWithYield(
     1,
     options.yieldAfterCodeUnits ?? CLIPBOARD_TEXT_MEASURE_YIELD_CODE_UNITS
   )
-  const yieldToEventLoop = options.yieldToEventLoop ?? defaultClipboardTextMeasureYield
+  const yieldBetweenBatches = options.yieldToEventLoop ?? yieldToEventLoop
   let nextYieldAt = yieldAfterCodeUnits
   let byteLength = 0
 
@@ -67,7 +69,7 @@ export async function measureClipboardTextByteLengthWithYield(
       index += 1
     }
     if (index >= nextYieldAt) {
-      await yieldToEventLoop()
+      await yieldBetweenBatches()
       nextYieldAt = index + yieldAfterCodeUnits
     }
   }
@@ -183,7 +185,3 @@ function getUtf8ByteLengthForCodePoint(codePoint: number): number {
   }
   return 4
 }
-
-function defaultClipboardTextMeasureYield(): Promise<void> {
-  return new Promise((resolve) => setTimeout(resolve, 0))
-}

src/shared/event-loop-yield.test.ts

@@ -0,0 +1,42 @@
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { yieldToEventLoop } from './event-loop-yield'
+
+afterEach(() => {
+  vi.unstubAllEnvs()
+  vi.unstubAllGlobals()
+})
+
+describe('yieldToEventLoop', () => {
+  it('uses setImmediate in Node runtimes', async () => {
+    const scheduleImmediate = vi.fn((callback: () => void) => queueMicrotask(callback))
+    vi.stubEnv('VITEST', 'false')
+    vi.stubGlobal('window', undefined)
+    vi.stubGlobal('setImmediate', scheduleImmediate)
+
+    await yieldToEventLoop()
+
+    expect(scheduleImmediate).toHaveBeenCalledOnce()
+  })
+
+  it('posts one macrotask per renderer yield', async () => {
+    const postMessage = vi.fn()
+    vi.stubEnv('VITEST', 'false')
+    vi.stubGlobal('window', {})
+    vi.stubGlobal(
+      'MessageChannel',
+      class {
+        port1: { onmessage: ((event: MessageEvent) => void) | null } = { onmessage: null }
+        port2 = {
+          postMessage: (data: unknown): void => {
+            postMessage(data)
+            queueMicrotask(() => this.port1.onmessage?.({ data } as MessageEvent))
+          }
+        }
+      }
+    )
+
+    await Promise.all([yieldToEventLoop(), yieldToEventLoop()])
+
+    expect(postMessage).toHaveBeenCalledTimes(2)
+  })
+})

src/shared/event-loop-yield.ts

@@ -0,0 +1,55 @@
+type ImmediateGlobal = typeof globalThis & {
+  setImmediate?: (callback: () => void) => unknown
+}
+
+const pendingRendererYields: (() => void)[] = []
+let nextRendererYieldIndex = 0
+let rendererYieldChannel: MessageChannel | null = null
+
+function isVitestEnvironment(): boolean {
+  return typeof process !== 'undefined' && process.env?.VITEST === 'true'
+}
+
+function getRendererYieldChannel(): MessageChannel {
+  if (!rendererYieldChannel) {
+    rendererYieldChannel = new globalThis.MessageChannel()
+    rendererYieldChannel.port1.onmessage = () => {
+      const resolve = pendingRendererYields[nextRendererYieldIndex]
+      if (!resolve) {
+        return
+      }
+      nextRendererYieldIndex += 1
+      if (nextRendererYieldIndex === pendingRendererYields.length) {
+        pendingRendererYields.length = 0
+        nextRendererYieldIndex = 0
+      }
+      resolve()
+    }
+  }
+  return rendererYieldChannel
+}
+
+export function yieldToEventLoop(): Promise<void> {
+  return new Promise((resolve) => {
+    // Vitest fake timers cannot advance MessageChannel tasks.
+    if (isVitestEnvironment()) {
+      globalThis.setTimeout(resolve, 0)
+      return
+    }
+
+    const setImmediate = (globalThis as ImmediateGlobal).setImmediate
+    if (typeof window === 'undefined' && setImmediate) {
+      setImmediate(resolve)
+      return
+    }
+
+    if (typeof globalThis.MessageChannel === 'function') {
+      // Posted tasks avoid Chromium's nested-timer clamp while still yielding to input and paint.
+      pendingRendererYields.push(resolve)
+      getRendererYieldChannel().port2.postMessage(undefined)
+      return
+    }
+
+    globalThis.setTimeout(resolve, 0)
+  })
+}