test(runtime): enforce ANSI preview-final pre-filter invariant (#27376)

Follow-up to #27344. isTerminalPreviewLineControl now gates on
isTerminalPreviewLineControlFinal, so the normalizeTerminalChunk
pre-filter cannot drop a final the predicate accepts. Add a test over
every CSI final 0x40-0x7e and replace String.prototype.charCodeAt spies
with behavior assertions.

src/main/runtime/terminal-ansi-normalization.ts

@@ -197,7 +197,7 @@ function trimPendingAnsiControl(value: string): string {
   return `${introducer}${value.slice(-suffixBudget)}`
 }
 
-/** CSI controls that affect retained preview rows. */
+/** CSI finals that affect retained preview rows; the sole gate for isTerminalPreviewLineControl. */
 function isTerminalPreviewLineControlFinal(code: number): boolean {
   return (
     code === 0x4b ||
@@ -214,12 +214,16 @@ function isTerminalPreviewLineControl(parsed: {
   params: string
   firstParam: number | null
 }): boolean {
-  if (!hasCanonicalNumericCsiParams(parsed.params)) {
+  // Why: normalizeTerminalChunk pre-filters on this predicate, so any final accepted here must pass it first.
+  if (
+    !isTerminalPreviewLineControlFinal(parsed.final.charCodeAt(0)) ||
+    !hasCanonicalNumericCsiParams(parsed.params)
+  ) {
     return false
   }
   if (parsed.final === 'K') {
     const mode = parsed.firstParam ?? 0
     return mode === 0 || mode === 1 || mode === 2
   }
-  return isTerminalPreviewLineControlFinal(parsed.final.charCodeAt(0))
+  return true
 }

src/main/runtime/terminal-vertical-control-scan.test.ts

@@ -1,4 +1,4 @@
-import { describe, expect, it, vi } from 'vitest'
+import { describe, expect, it } from 'vitest'
 import {
   containsTerminalVerticalLineControl,
   normalizeTerminalChunk
@@ -6,20 +6,42 @@ import {
 import { appendNormalizedToTailBuffer } from './terminal-tail-buffer'
 
 describe('terminal vertical-control scanning', () => {
-  it('scans retained CSI parameters once while normalizing', () => {
+  it('keeps a line control with long retained CSI parameters', () => {
     const input = `\x1b[${'1;'.repeat(2048)}A`
-    const charCodeAt = vi.spyOn(String.prototype, 'charCodeAt')
-    let normalized: ReturnType<typeof normalizeTerminalChunk>
-    let inspections: number
-    try {
-      normalized = normalizeTerminalChunk(input)
-      inspections = charCodeAt.mock.calls.length
-    } finally {
-      charCodeAt.mockRestore()
-    }
+    expect(normalizeTerminalChunk(input)).toEqual({
+      text: input,
+      pendingAnsi: ''
+    })
+  })
 
-    expect(normalized).toEqual({ text: input, pendingAnsi: '' })
-    expect(inspections).toBeLessThanOrEqual(input.length + 8)
+  it('retains exactly the preview line-control CSI finals', () => {
+    const retained: string[] = []
+    for (let code = 0x40; code <= 0x7e; code += 1) {
+      const final = String.fromCharCode(code)
+      for (const params of ['', '2']) {
+        const control = `\x1b[${params}${final}`
+        const { text } = normalizeTerminalChunk(`a${control}b`)
+        if (text === `a${control}b`) {
+          retained.push(control)
+        } else {
+          expect(text).toBe('ab')
+        }
+      }
+    }
+    expect(retained).toEqual([
+      '\x1b[A',
+      '\x1b[2A',
+      '\x1b[C',
+      '\x1b[2C',
+      '\x1b[D',
+      '\x1b[2D',
+      '\x1b[G',
+      '\x1b[2G',
+      '\x1b[K',
+      '\x1b[2K',
+      '\x1b[`',
+      '\x1b[2`'
+    ])
   })
 
   it('preserves printable spans and carried controls across chunk boundaries', () => {
@@ -35,19 +57,8 @@ describe('terminal vertical-control scanning', () => {
     ['plain', 'log output '.repeat(8192), false],
     ['nonvertical CSI', `\x1b[31m${'log output '.repeat(8192)}\x1b[0m`, false],
     ['vertical CSI', `${'漢字😀 output '.repeat(8192)}\x1b[2A`, true]
-  ] as const)('bounds code-unit inspections on %s output', (_name, input, expected) => {
-    const charCodeAt = vi.spyOn(String.prototype, 'charCodeAt')
-    let actual: boolean
-    let inspections: number
-    try {
-      actual = containsTerminalVerticalLineControl(input)
-      inspections = charCodeAt.mock.calls.length
-    } finally {
-      charCodeAt.mockRestore()
-    }
-
-    expect(actual).toBe(expected)
-    expect(inspections).toBeLessThan(16)
+  ] as const)('detects vertical controls in %s output', (_name, input, expected) => {
+    expect(containsTerminalVerticalLineControl(input)).toBe(expected)
   })
 
   it.each([