refactor(native-chat): tidy what the diff-card work left behind

The copy text is joined from every row of the diff, which a collapsed card
renders none of, and it was rebuilt on every render to seed a prop. It is
memoized on the rows, matching how the run memoizes its edit model.

The two scanners that read patch text kept the same file-section alternation
verbatim, so they could drift apart while both looking correct; there is one
definition now, beside the header-pair rule that already lives there.

Also: the row that marks a break between regions is built in one place, so it
is no longer exported; the move destination in the envelope reader was a
function-wide binding written and read within one iteration, which read as if
a move carried between sections; and a test comment named the wrong mechanism
for keeping a card collapsed.

Adds the missing pin on what the copy affordance actually copies.

src/renderer/src/components/native-chat/NativeChatDiffCard.tsx

@@ -1,4 +1,4 @@
-import { useState } from 'react'
+import { useMemo, useState } from 'react'
 import { ChevronRight, FilePlus2, FileMinus2, FilePen } from 'lucide-react'
 import { cn } from '@/lib/utils'
 import { translate } from '@/i18n/i18n'
@@ -38,8 +38,8 @@ function baseName(path: string): string {
   return path.split(/[\\/]/).at(-1) || path
 }
 
-function patchText(file: NativeChatEditFile): string {
-  return file.lines
+function patchText(lines: readonly NativeChatEditLine[]): string {
+  return lines
     .filter((line) => line.kind !== 'gap')
     .map((line) => `${line.kind === 'add' ? '+' : line.kind === 'del' ? '-' : ' '}${line.text}`)
     .join('\n')
@@ -117,6 +117,9 @@ export function NativeChatDiffCard({
   initiallyExpanded?: boolean
 }): React.JSX.Element {
   const [expanded, setExpanded] = useState(initiallyExpanded)
+  // Joining every row to seed the copy button is the card's most expensive
+  // work, and a collapsed card renders none of those rows.
+  const copyText = useMemo(() => patchText(file.lines), [file.lines])
   const hasBody = file.lines.length > 0
   const widest = file.lineNumbersKnown
     ? file.lines.reduce((max, line) => Math.max(max, unifiedLineNumber(line) ?? 0), 0)
@@ -171,7 +174,7 @@ export function NativeChatDiffCard({
           </span>
         ) : null}
         <NativeChatCopyButton
-          text={patchText(file)}
+          text={copyText}
           label={translate('components.native-chat.tool.copyDiff', 'Copy diff')}
           className="ml-auto shrink-0"
         />

src/renderer/src/components/native-chat/NativeChatToolRun.test.tsx

@@ -2,8 +2,8 @@
 
 import '@testing-library/jest-dom/vitest'
 
-import { cleanup, render, screen } from '@testing-library/react'
-import { afterEach, describe, expect, it } from 'vitest'
+import { cleanup, fireEvent, render, screen } from '@testing-library/react'
+import { afterEach, describe, expect, it, vi } from 'vitest'
 import type { AgentJournalRenderItem } from '../../../../shared/agent-session-journal-types'
 import type { NativeChatBlock } from '../../../../shared/native-chat-types'
 import { projectStructuredItemToNativeChat } from '../../../../shared/structured-agent-session-projection'
@@ -196,13 +196,42 @@ describe('NativeChatToolRun', () => {
       { type: 'tool-result', output: '@@ -1,3 +1,3 @@\n ctx\n-was\n+now\n… (48210 bytes)' }
     ]
 
-    // expandSignal false leaves every card's body closed.
+    // A defined expandOverride opens the run while leaving each card closed.
     render(<NativeChatToolRun blocks={blocks} expandSignal={false} expandOverride />)
 
     expect(screen.getByText('Diff truncated')).toBeInTheDocument()
     expect(screen.queryByText('was')).toBeNull()
   })
 
+  it('copies the diff as signed rows, with the region breaks left out', () => {
+    const writeClipboardText = vi.fn()
+    Object.assign(window, { api: { ui: { writeClipboardText } } })
+    const blocks: NativeChatBlock[] = [
+      {
+        type: 'tool-call',
+        name: 'Edit',
+        input: { file_path: '/repo/a.ts' },
+        state: 'completed'
+      },
+      {
+        type: 'tool-result',
+        output: 'ok',
+        editPatch: {
+          filePath: '/repo/a.ts',
+          hunks: [
+            { oldStart: 1, oldLines: 2, newStart: 1, newLines: 2, lines: [' ctx', '-was', '+now'] },
+            { oldStart: 90, oldLines: 1, newStart: 90, newLines: 1, lines: ['+tail'] }
+          ]
+        }
+      }
+    ]
+
+    render(<NativeChatToolRun blocks={blocks} expandSignal />)
+    fireEvent.click(screen.getByRole('button', { name: 'Copy diff' }))
+
+    expect(writeClipboardText).toHaveBeenCalledWith(' ctx\n-was\n+now\n+tail')
+  })
+
   it('keeps a grouped active run to one stable row showing only the latest tool', () => {
     const blocks: NativeChatBlock[] = [
       { type: 'tool-call', name: 'shell', input: { command: 'date' }, state: 'completed' },

src/shared/native-chat-begin-patch.ts

@@ -108,7 +108,6 @@ function envelopeArgument(
 /** Splits a `*** Begin Patch` envelope into one entry per file it touches. */
 export function editFilesFromBeginPatch(envelope: string): NativeChatEditFile[] {
   const sections: { kind: 'Add' | 'Update' | 'Delete'; path: string; body: string[] }[] = []
-  let movePath: string | null = null
   const moves = new Map<number, string>()
 
   // Split on both newline forms once, so every marker below can be matched
@@ -125,8 +124,7 @@ export function editFilesFromBeginPatch(envelope: string): NativeChatEditFile[]
     }
     const move = MOVE_HEADER.exec(raw)
     if (move && sections.length > 0) {
-      movePath = move[1]!.trim()
-      moves.set(sections.length - 1, movePath)
+      moves.set(sections.length - 1, move[1]!.trim())
       continue
     }
     if (raw === BEGIN || raw === END || CONTROL_LINE.test(raw) || sections.length === 0) {

src/shared/native-chat-diff.ts

@@ -14,8 +14,11 @@ const DIFF_TRUNCATED_LINE: NativeChatDiffLine = {
 }
 
 const HUNK_HEADER = /^@@ -\d+(?:,\d+)? \+\d+(?:,\d+)? @@/
-// Lines that open a new file section, so any hunk before them has ended.
-const FILE_SECTION_START =
+/** Lines that open a new file section, so any hunk before them has ended.
+ *  `--- `/`+++ ` are deliberately absent: inside a hunk they are content — a
+ *  removed `-- comment` is emitted as `--- comment` — so they go through
+ *  `isFileHeaderPair` instead. */
+export const FILE_SECTION_START =
   /^(?:diff |index |old mode |new mode |new file mode |deleted file mode |similarity index |dissimilarity index |rename |copy |Binary files )/
 // Markdown thematic break or YAML document separator, not a marker.
 const BARE_RULE = /^(?:-{3,}|\+{3,})$/

src/shared/native-chat-edit-model.ts

@@ -54,7 +54,7 @@ export function splitEditContent(content: string): EditContentLines {
 }
 
 /** The break between two regions of a file. Carries no text and no position. */
-export function editGapLine(): NativeChatEditLine {
+function editGapLine(): NativeChatEditLine {
   return { kind: 'gap', text: '', oldLineNumber: null, newLineNumber: null }
 }
 

src/shared/native-chat-unified-patch.ts

@@ -1,13 +1,7 @@
-import { isFileHeaderPair } from './native-chat-diff'
+import { FILE_SECTION_START, isFileHeaderPair } from './native-chat-diff'
 import { pushEditGap, splitEditContent, type NativeChatEditLine } from './native-chat-edit-model'
 
 const HUNK_RANGES = /^@@+ -(\d+)(?:,(\d+))? \+(\d+)(?:,(\d+))? @@/
-/** Structural lines that open a file section, so any hunk before them has ended.
- *  `--- `/`+++ ` are deliberately absent: inside a hunk they are content — a
- *  removed `-- comment` is emitted as `--- comment` — so they go through
- *  `isFileHeaderPair` instead. */
-const FILE_SECTION =
-  /^(?:diff |index |old mode |new mode |new file mode |deleted file mode |similarity index |dissimilarity index |rename |copy |Binary files )/
 
 export type UnifiedPatchLines = {
   lines: NativeChatEditLine[]
@@ -59,7 +53,7 @@ export function editLinesFromUnifiedPatch(
       index += 1
       continue
     }
-    if (FILE_SECTION.test(raw)) {
+    if (FILE_SECTION_START.test(raw)) {
       inHunk = false
       continue
     }
@@ -186,7 +180,7 @@ export function unifiedPatchSections(text: string): {
     }
     if (raw.startsWith('@@')) {
       inHunk = true
-    } else if (FILE_SECTION.test(raw)) {
+    } else if (FILE_SECTION_START.test(raw)) {
       inHunk = false
     }
     current ??= open()