test: add perf test for viewport pan sweep GC churn (#10479)

## Summary

Adds a `@perf` test to establish a baseline for viewport panning GC
churn on large graphs.

## Changes

- **What**: New `large graph viewport pan sweep` perf test that pans
aggressively back and forth across a 245-node graph, forcing many nodes
to cross the viewport boundary. Measures style recalcs, forced layouts,
task duration, heap delta, and DOM node count.

## Review Focus

This is **PR 1 of 2** (perf-fix-with-proof pattern). The fix (viewport
culling) will follow in a separate PR once this baseline is established
on main. CI will then show the delta proving the improvement.

The test uses 120 steps out + 120 steps back at 8px/step = ~960px total
displacement, enough to sweep across a significant portion of the large
graph layout.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10479-test-add-perf-test-for-viewport-pan-sweep-GC-churn-32d6d73d365081cc9f15fe3d5890675d)
by [Unito](https://www.unito.io)

browser_tests/fixtures/helpers/CanvasHelper.ts

@@ -169,6 +169,39 @@ export class CanvasHelper {
     })
   }
 
+  /**
+   * Pan the canvas back and forth in a sweep pattern using middle-mouse drag.
+   * Each step advances one animation frame, giving per-frame measurement
+   * granularity for performance tests.
+   */
+  async panSweep(options?: {
+    steps?: number
+    dx?: number
+    dy?: number
+  }): Promise<void> {
+    const { steps = 120, dx = 8, dy = 3 } = options ?? {}
+    const box = await this.canvas.boundingBox()
+    if (!box) throw new Error('Canvas bounding box not available')
+
+    const centerX = box.x + box.width / 2
+    const centerY = box.y + box.height / 2
+    await this.page.mouse.move(centerX, centerY)
+    await this.page.mouse.down({ button: 'middle' })
+
+    // Sweep forward
+    for (let i = 0; i < steps; i++) {
+      await this.page.mouse.move(centerX + i * dx, centerY + i * dy)
+      await this.nextFrame()
+    }
+    // Sweep back
+    for (let i = steps; i > 0; i--) {
+      await this.page.mouse.move(centerX + i * dx, centerY + i * dy)
+      await this.nextFrame()
+    }
+
+    await this.page.mouse.up({ button: 'middle' })
+  }
+
   async disconnectEdge(): Promise<void> {
     await this.dragAndDrop(
       DefaultGraphPositions.clipTextEncodeNode1InputSlot,

browser_tests/helpers/perfReporter.ts

@@ -12,6 +12,38 @@ export interface PerfReport {
 
 const TEMP_DIR = join('test-results', 'perf-temp')
 
+type MeasurementField = keyof PerfMeasurement
+
+const FIELD_FORMATTERS: Record<string, (m: PerfMeasurement) => string> = {
+  styleRecalcs: (m) => `${m.styleRecalcs} recalcs`,
+  layouts: (m) => `${m.layouts} layouts`,
+  taskDurationMs: (m) => `${m.taskDurationMs.toFixed(1)}ms task`,
+  layoutDurationMs: (m) => `${m.layoutDurationMs.toFixed(1)}ms layout`,
+  frameDurationMs: (m) => `${m.frameDurationMs.toFixed(1)}ms/frame`,
+  totalBlockingTimeMs: (m) => `TBT=${m.totalBlockingTimeMs.toFixed(0)}ms`,
+  durationMs: (m) => `${m.durationMs.toFixed(0)}ms total`,
+  heapDeltaBytes: (m) => `heap Δ${(m.heapDeltaBytes / 1024).toFixed(0)}KB`,
+  domNodes: (m) => `DOM Δ${m.domNodes}`,
+  heapUsedBytes: (m) => `heap ${(m.heapUsedBytes / 1024 / 1024).toFixed(1)}MB`
+}
+
+/**
+ * Log a perf measurement to the console in a consistent format.
+ * Fields are formatted automatically based on their type.
+ */
+export function logMeasurement(
+  label: string,
+  m: PerfMeasurement,
+  fields: MeasurementField[]
+) {
+  const parts = fields.map((f) => {
+    const formatter = FIELD_FORMATTERS[f]
+    if (formatter) return formatter(m)
+    return `${f}=${m[f]}`
+  })
+  console.log(`${label}: ${parts.join(', ')}`)
+}
+
 export function recordMeasurement(m: PerfMeasurement) {
   mkdirSync(TEMP_DIR, { recursive: true })
   const filename = `${m.name}-${Date.now()}.json`

browser_tests/tests/performance.spec.ts

@@ -1,7 +1,7 @@
 import { expect } from '@playwright/test'
 
 import { comfyPageFixture as test } from '../fixtures/ComfyPage'
-import { recordMeasurement } from '../helpers/perfReporter'
+import { logMeasurement, recordMeasurement } from '../helpers/perfReporter'
 
 test.describe('Performance', { tag: ['@perf'] }, () => {
   test('canvas idle style recalculations', async ({ comfyPage }) => {
@@ -186,6 +186,22 @@ test.describe('Performance', { tag: ['@perf'] }, () => {
     )
   })
 
+  test('large graph viewport pan sweep', async ({ comfyPage }) => {
+    await comfyPage.workflow.loadWorkflow('large-graph-workflow')
+
+    await comfyPage.perf.startMeasuring()
+    await comfyPage.canvasOps.panSweep()
+
+    const measurement = await comfyPage.perf.stopMeasuring('viewport-pan-sweep')
+    recordMeasurement(measurement)
+    logMeasurement('Viewport pan sweep', measurement, [
+      'styleRecalcs',
+      'layouts',
+      'taskDurationMs',
+      'heapDeltaBytes',
+      'domNodes'
+    ])
+  })
   test('subgraph DOM widget clipping during node selection', async ({
     comfyPage
   }) => {