bench: time only drawBox in the box benchmark (#1643)

Version 2 of JS Buffer/draw-box-titled-scissored took a checked buffer
lease on every iteration to observe one cell. The timed operation
became drawBox plus three FFI calls and a new external ArrayBuffer.
On Bun the lease was about half of each operation. On Node it made
batch times vary by 5% to 9% and failed the 5% inner RSD gate.

Version 3 times only drawBox. Each batch checks the last drawn
variant once, outside the timed loop, then clears the signal cell so
a batch without draws cannot pass on an earlier result.

packages/core/src/benchmark/js-benchmark-cases.ts

@@ -205,7 +205,9 @@ const BOX_BORDER_FG = RGBA.fromInts(96, 165, 250)
 const BOX_TITLE_FGS = [RGBA.fromInts(250, 204, 21), RGBA.fromInts(52, 211, 153)] as const
 const BOX_TOP_TITLES = [" Alpha ", " Omega "] as const
 const BOX_BOTTOM_TITLES = [" Ready ", " Busy! "] as const
-const BOX_SIGNAL_INDEX = 2 * BOX_BUFFER_WIDTH + 37
+const BOX_SIGNAL_X = 37
+const BOX_SIGNAL_Y = 2
+const BOX_SIGNAL_INDEX = BOX_SIGNAL_Y * BOX_BUFFER_WIDTH + BOX_SIGNAL_X
 const BOX_SIGNAL_CHARS = [65, 79] as const
 const BOX_VARIANTS: readonly Parameters<OptimizedBuffer["drawBox"]>[0][] = BOX_TOP_TITLES.map((title, index) => ({
   x: 2,
@@ -228,8 +230,8 @@ function directBoxDrawingCase(): BenchmarkCase {
   return {
     category: "JS Buffer",
     name: "draw-box-titled-scissored",
-    // Version 2 includes a checked lease for each cell observation.
-    workload_version: 2,
+    // Version 3 times only drawBox. Each batch checks the last draw outside the timed loop.
+    workload_version: 3,
     parameters: {
       buffer_width: BOX_BUFFER_WIDTH,
       buffer_height: BOX_BUFFER_HEIGHT,
@@ -258,17 +260,14 @@ function directBoxDrawingCase(): BenchmarkCase {
 
         buffer.clear(BOX_CLEAR_BG)
         buffer.pushScissorRect(0, 0, 72, BOX_BUFFER_HEIGHT)
-        let observedSignal = 0
-        let expectedSignal = 0
+        let variant = 0
         let completed = 0
         let validated = 0
 
         return {
           run(iteration) {
-            const variant = iteration & 1
+            variant = iteration & 1
             buffer.drawBox(BOX_VARIANTS[variant]!)
-            observedSignal = (observedSignal + buffer.withBuffers((cells) => cells.char[BOX_SIGNAL_INDEX]!)) | 0
-            expectedSignal = (expectedSignal + BOX_SIGNAL_CHARS[variant]!) | 0
             completed++
           },
           validateBatch(iterations) {
@@ -276,9 +275,13 @@ function directBoxDrawingCase(): BenchmarkCase {
             if (actual !== iterations) {
               throw new Error(`draw-box-titled-scissored: completed ${actual} operations, expected ${iterations}`)
             }
-            if (observedSignal !== expectedSignal) {
-              throw new Error(`draw-box-titled-scissored: observation ${observedSignal}, expected ${expectedSignal}`)
+            const observed = buffer.withBuffers((cells) => cells.char[BOX_SIGNAL_INDEX]!)
+            const expected = BOX_SIGNAL_CHARS[variant]!
+            if (observed !== expected) {
+              throw new Error(`draw-box-titled-scissored: observation ${observed}, expected ${expected}`)
             }
+            // Clear the signal so a batch without draws cannot pass on the previous batch's cell.
+            buffer.setCell(BOX_SIGNAL_X, BOX_SIGNAL_Y, " ", BOX_BORDER_FG, BOX_CLEAR_BG)
             validated = completed
           },
           teardown() {

packages/core/src/benchmark/js-benchmark-harness.test.ts

@@ -71,8 +71,8 @@ describe("statistics and manifest", () => {
       "JS Text/text-buffer-word-wrap-measure",
       "JS Buffer/draw-box-titled-scissored",
     ])
-    expect(manifest.cases.find(({ name }) => name === "draw-box-titled-scissored")?.workload_version).toBe(2)
-    expect(manifestHash(manifest)).toBe("sha256:a046b0f302d33ad7eacdbe6e0b951f2a60c7b43863acd3f3bfd06c611902f01e")
+    expect(manifest.cases.find(({ name }) => name === "draw-box-titled-scissored")?.workload_version).toBe(3)
+    expect(manifestHash(manifest)).toBe("sha256:41cc6f0856b059d6b77f0575f7cdd7c297ddd9e59b55185f3b7fc1ef4ce7e1d7")
     expect(manifestHash({ ...manifest, protocol_version: 2 })).not.toBe(manifestHash(manifest))
   })
 
@@ -314,14 +314,17 @@ test("canonical cases perform and validate work across consecutive batches", asy
   }
 })
 
-test("canonical box drawing validation rejects a no-op", async () => {
+test("canonical box drawing validation rejects a batch of no-ops", async () => {
   const benchmark = defaultBenchmarkCases.find(({ name }) => name === "draw-box-titled-scissored")!
   const runtime = await benchmark.setup()
   const drawBox = OptimizedBuffer.prototype.drawBox
-  OptimizedBuffer.prototype.drawBox = () => {}
   try {
     runtime.run(0)
-    expect(() => runtime.validateBatch(1)).toThrow("draw-box-titled-scissored: observation")
+    runtime.validateBatch(1)
+    OptimizedBuffer.prototype.drawBox = () => {}
+    runtime.run(1)
+    runtime.run(2)
+    expect(() => runtime.validateBatch(2)).toThrow("draw-box-titled-scissored: observation")
   } finally {
     OptimizedBuffer.prototype.drawBox = drawBox
     await runtime.teardown()