fix(read/edit): make compact line prefix unambiguous for tab-indented files (#613)

src/utils/file.test.ts

@@ -0,0 +1,51 @@
+import { afterEach, describe, expect, mock, test } from 'bun:test'
+
+async function importFileModuleWithKillswitchEnabled(
+  killswitchEnabled: boolean,
+) {
+  mock.module('../services/analytics/growthbook.js', () => ({
+    getFeatureValue_CACHED_MAY_BE_STALE: () => killswitchEnabled,
+  }))
+
+  return import(`./file.js?ts=${Date.now()}-${Math.random()}`)
+}
+
+afterEach(() => {
+  mock.restore()
+})
+
+describe('addLineNumbers', () => {
+  test('uses unambiguous arrow compact prefix and preserves leading tabs', async () => {
+    const { addLineNumbers } = await importFileModuleWithKillswitchEnabled(false)
+
+    const result = addLineNumbers({
+      content: '\tfirst\n\t\tsecond',
+      startLine: 41,
+    })
+
+    expect(result).toBe('41→\tfirst\n42→\t\tsecond')
+  })
+
+  test('keeps padded arrow format when compact mode is disabled', async () => {
+    const { addLineNumbers } = await importFileModuleWithKillswitchEnabled(true)
+
+    const result = addLineNumbers({
+      content: 'alpha\nbeta',
+      startLine: 1,
+    })
+
+    expect(result).toBe('     1→alpha\n     2→beta')
+  })
+})
+
+describe('stripLineNumberPrefix', () => {
+  test('strips compact arrow, padded arrow, and legacy tab prefixes', async () => {
+    const { stripLineNumberPrefix } = await importFileModuleWithKillswitchEnabled(
+      false,
+    )
+
+    expect(stripLineNumberPrefix('41→\tfirst')).toBe('\tfirst')
+    expect(stripLineNumberPrefix('     2→beta')).toBe('beta')
+    expect(stripLineNumberPrefix('7\t\tlegacy-tab')).toBe('\tlegacy-tab')
+  })
+})