fix: avoid quadratic trimming during fullscreen terminal redraws (#19214)

config/reliability-gates.jsonc

@@ -10,6 +10,75 @@
     }
   },
   "gates": [
+    {
+      "id": "terminal-performance.padded-fullscreen-redraw",
+      "title": "Fullscreen redraw padding does not stall terminal delivery",
+      "maturity": "experimental",
+      "protection": "partial",
+      "owner": "terminal-runtime",
+      "layer": "runtime-unit-and-electron-cdp",
+      "surfaces": ["terminal transcript preview", "fullscreen TUI scrolling"],
+      "platforms": ["macos", "linux", "windows"],
+      "providers": ["local", "daemon", "ssh", "remote-runtime"],
+      "coveredPlatforms": ["macos"],
+      "coveredProviders": ["local", "daemon"],
+      "coverageNotes": "The trim operation is platform-independent and preserves the same spaces/tabs policy for all providers. Real Pi 0.84.2 was exercised in a hidden macOS Electron renderer through CDP using a folder workspace.",
+      "motivatingLinks": ["https://github.com/stablyai/orca/issues/14770"],
+      "invariant": "Transcript preview trimming preserves internal whitespace and terminal read contents without quadratic main-process work on padded fullscreen redraws.",
+      "oracle": "Preserve 32,000 spaces before a marker while trimming trailing spaces/tabs in both retained-row and carried-prefix redraw paths; four redraws must finish within 500 ms. Existing tail equivalence tests preserve cursor, retention, and pagination behavior.",
+      "commands": [
+        "ORCA_BACKGROUND_LAUNCH=1 pnpm test src/main/runtime/terminal-tail-whitespace.test.ts src/main/runtime/terminal-tail-buffer.test.ts src/main/runtime/retained-tail-redraw-window.equivalence.test.ts"
+      ],
+      "testFiles": [
+        "src/main/runtime/terminal-tail-whitespace.test.ts",
+        "src/main/runtime/terminal-tail-buffer.test.ts",
+        "src/main/runtime/retained-tail-redraw-window.equivalence.test.ts"
+      ],
+      "assertionRefs": [
+        {
+          "file": "src/main/runtime/terminal-tail-whitespace.test.ts",
+          "assertions": [
+            "handles padded redraws across %i retained rows without stalling",
+            "preserves terminal text while trimming spaces and tabs: %j"
+          ]
+        }
+      ],
+      "evidenceRuns": [
+        {
+          "date": "2026-09-06",
+          "runner": "local",
+          "platform": "macos",
+          "command": "ORCA_BACKGROUND_LAUNCH=1 pnpm test src/main/runtime/terminal-tail-whitespace.test.ts src/main/runtime/terminal-tail-buffer.test.ts src/main/runtime/retained-tail-redraw-window.equivalence.test.ts",
+          "result": "passed",
+          "durationSeconds": 3.96,
+          "summary": "17 tests passed. Before the fix both padding budget cases failed, taking approximately 1.7 seconds each."
+        }
+      ],
+      "runtimeBudget": {
+        "p95Seconds": 30,
+        "scope": "Three unit test files; padding cases allow 500 ms for four redraws."
+      },
+      "flakeHistory": {
+        "status": "not-started",
+        "evidence": "Initial local red/green validation; no CI soak history yet."
+      },
+      "redGreenEvidence": {
+        "status": "complete",
+        "evidence": "Both padding budget cases fail with regex trimming and pass with the existing linear trim. A 60-event CDP wheel stream in Pi fullscreen had about 2.1 seconds of output tail before the fix and 14 ms after rebuilding."
+      },
+      "performanceBudget": {
+        "required": true,
+        "evidence": "The main CPU profile attributed 3.1 seconds to redraw-row whitespace trimming. Reusing the linear trim adds no timers, caches, provider calls, or output dropping."
+      },
+      "knownGaps": [
+        "The user manually compared the fixed dev app with production and confirmed improved responsiveness. A live Terminal.app comparison was not exercised; timing measurements used CDP wheel events.",
+        "Linux, Windows, and live SSH rendering were not exercised; the shared trimming behavior is covered by unit tests."
+      ],
+      "promotionCriteria": [
+        "Complete CI soak requirements and retain the padding budget and tail equivalence oracles."
+      ],
+      "demotionRule": "Keep experimental until CI soak is stable; investigate any budget failure without weakening transcript preservation."
+    },
     {
       "id": "ssh.localhost-terminal-agent-hooks",
       "title": "Localhost SSH terminal and agent hooks reach the owning pane",

src/main/runtime/terminal-tail-buffer.ts

@@ -293,7 +293,7 @@ function appendNormalizedToMultilineTailBuffer(
       const line = rewritten[index]!
       const lastChar = line.charCodeAt(line.length - 1)
       if (lastChar === 32 || lastChar === 9) {
-        rewritten[index] = line.replace(/[ \t]+$/g, '')
+        rewritten[index] = trimTerminalLineRight(line)
       }
     }
     for (const line of windowed.lines) {

src/main/runtime/terminal-tail-redraw-buffer.ts

@@ -185,7 +185,7 @@ function finalizeRetainedTerminalRows(
   newlyCompletedLines: string[]
 } {
   let truncated = initialTruncated
-  let retainedRows = rows.map((row) => ({ ...row, text: row.text.replace(/[ \t]+$/g, '') }))
+  let retainedRows = rows.map((row) => ({ ...row, text: trimTerminalLineRight(row.text) }))
 
   if (retainedRows.length > MAX_TAIL_LINES + 1) {
     const removeCount = retainedRows.length - (MAX_TAIL_LINES + 1)

src/main/runtime/terminal-tail-whitespace.test.ts

@@ -0,0 +1,32 @@
+import { performance } from 'node:perf_hooks'
+import { describe, expect, it } from 'vitest'
+import { appendNormalizedToTailBuffer } from './terminal-tail-buffer'
+import { trimTerminalLineRight } from './terminal-tail-line-controls'
+
+describe('terminal redraw whitespace', () => {
+  it.each([
+    ['hello \t', 'hello'],
+    [' \thello \t world \t', ' \thello \t world'],
+    [' \t', ''],
+    ['hello\u00a0 \t', 'hello\u00a0'],
+    ['hello\n', 'hello\n']
+  ])('preserves terminal text while trimming spaces and tabs: %j', (input, expected) => {
+    expect(trimTerminalLineRight(input)).toBe(expected)
+  })
+
+  it.each([2, 20])('handles padded redraws across %i retained rows without stalling', (rows) => {
+    const padded = `${' '.repeat(32_000)}marker \t`
+    const previousLines = Array.from({ length: rows }, (_, index) =>
+      index === 0 ? padded : `row ${index}`
+    )
+    const start = performance.now()
+    let result: ReturnType<typeof appendNormalizedToTailBuffer> | undefined
+    for (let frame = 0; frame < 4; frame += 1) {
+      result = appendNormalizedToTailBuffer(previousLines, 'footer', '\x1b[1A\rupdated')
+    }
+    const elapsedMs = performance.now() - start
+    expect(result?.lines[0]).toBe(`${' '.repeat(32_000)}marker`)
+    // Interior padding made the trailing-whitespace regex backtrack quadratically.
+    expect(elapsedMs).toBeLessThan(500)
+  })
+})