fix(native-chat): stop diff colouring from misreading -- / ++ content lines as file headers

diffFromText skipped every line starting with --- / +++ as a file header, so a
deleted SQL/Lua '-- comment' (git emits '---<content>') or an added '++flag' fell
through to gray context with its marker still attached — and when it was the only
change, the two-marker gate dropped the coloured diff entirely.

Detect real headers structurally instead: an adjacent '--- <old>' / '+++ <new>'
pair outside any hunk. A hunk header or 'diff --git' line now also proves the text
is a diff, so a genuine single-line change renders while prose keeps the guard.

Co-authored-by: Orca <help@stably.ai>

src/renderer/src/components/native-chat/native-chat-diff.test.ts

@@ -60,4 +60,88 @@ describe('diffFromText', () => {
     expect(diff?.filter((l) => l.kind === 'add').map((l) => l.text)).toEqual(['new'])
     expect(diff?.filter((l) => l.kind === 'del').map((l) => l.text)).toEqual(['old'])
   })
+
+  it('treats a removed line whose content starts with -- as a deletion', () => {
+    const diff = diffFromText(
+      '@@ -1,4 +1,1 @@\n SELECT 1;\n--- legacy comment\n-SELECT 2;\n-SELECT 3;'
+    )
+    expect(diff).toEqual([
+      { kind: 'meta', text: '@@ -1,4 +1,1 @@' },
+      { kind: 'context', text: ' SELECT 1;' },
+      { kind: 'del', text: '-- legacy comment' },
+      { kind: 'del', text: 'SELECT 2;' },
+      { kind: 'del', text: 'SELECT 3;' }
+    ])
+  })
+
+  it('treats an added line whose content starts with ++ as an addition', () => {
+    const diff = diffFromText('@@ -1,1 +1,3 @@\n ctx\n+++flag\n+normal')
+    expect(diff?.filter((l) => l.kind === 'add').map((l) => l.text)).toEqual(['++flag', 'normal'])
+  })
+
+  it('classifies a C-style --i deletion inside a hunk', () => {
+    const diff = diffFromText('@@ -1,3 +1,2 @@\n int i;\n---i;\n-return i;')
+    expect(diff?.filter((l) => l.kind === 'del').map((l) => l.text)).toEqual(['--i;', 'return i;'])
+  })
+
+  it('keeps real file headers as meta in full git diff output', () => {
+    const diff = diffFromText(
+      [
+        'diff --git a/a.sql b/a.sql',
+        'index 9eff25c..33791bf 100644',
+        '--- a/a.sql',
+        '+++ b/a.sql',
+        '@@ -1,3 +1,2 @@',
+        ' SELECT 1;',
+        '--- legacy comment',
+        ' SELECT 2;'
+      ].join('\n')
+    )
+    expect(diff?.filter((l) => l.kind === 'meta').map((l) => l.text)).toEqual([
+      'diff --git a/a.sql b/a.sql',
+      'index 9eff25c..33791bf 100644',
+      '--- a/a.sql',
+      '+++ b/a.sql',
+      '@@ -1,3 +1,2 @@'
+    ])
+    expect(diff?.filter((l) => l.kind === 'del').map((l) => l.text)).toEqual(['-- legacy comment'])
+  })
+
+  it('detects the second file headers in a multi-file diff', () => {
+    const diff = diffFromText(
+      [
+        'diff --git a/a.sql b/a.sql',
+        '--- a/a.sql',
+        '+++ b/a.sql',
+        '@@ -1,2 +1,1 @@',
+        '--- legacy comment',
+        ' SELECT 2;',
+        'diff --git a/b.lua b/b.lua',
+        '--- a/b.lua',
+        '+++ b/b.lua',
+        '@@ -1,2 +1,1 @@',
+        '--- lua comment',
+        ' print(1)'
+      ].join('\n')
+    )
+    expect(diff?.filter((l) => l.kind === 'del').map((l) => l.text)).toEqual([
+      '-- legacy comment',
+      '-- lua comment'
+    ])
+  })
+
+  it('renders a single-line change once hunk headers prove it is a diff', () => {
+    expect(diffFromText('@@ -1,2 +1,1 @@\n SELECT 1;\n--- legacy comment')).not.toBeNull()
+    // Without diff structure the two-marker prose guard still applies.
+    expect(diffFromText('-- legacy comment')).toBeNull()
+  })
+
+  it('handles /dev/null headers for an added file', () => {
+    const diff = diffFromText('--- /dev/null\n+++ b/x.sql\n+-- new comment\n+SELECT 1;')
+    expect(diff?.filter((l) => l.kind === 'add').map((l) => l.text)).toEqual([
+      '-- new comment',
+      'SELECT 1;'
+    ])
+    expect(diff?.filter((l) => l.kind === 'del')).toEqual([])
+  })
 })

src/shared/native-chat-diff.ts

@@ -13,6 +13,39 @@ const DIFF_TRUNCATED_LINE: NativeChatDiffLine = {
   text: '… diff truncated …'
 }
 
+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 =
+  /^(?:diff |index |old mode |new mode |new file mode |deleted file mode |similarity index |dissimilarity index |rename |copy |Binary files )/
+
+/**
+ * Indices of the `--- <old>` / `+++ <new>` file headers. A bare `---`/`+++`
+ * prefix is not enough to spot one: a removed line whose content began with `--`
+ * (SQL/Lua `-- comment`, C `--i`) is emitted as `---<content>`. Real headers
+ * always come as an adjacent pair and never appear inside a hunk.
+ */
+function fileHeaderIndices(lines: string[]): Set<number> {
+  const indices = new Set<number>()
+  let inHunk = false
+  for (let index = 0; index < lines.length; index += 1) {
+    const line = lines[index] ?? ''
+    if (line.startsWith('@@')) {
+      inHunk = true
+      continue
+    }
+    if (FILE_SECTION_START.test(line)) {
+      inHunk = false
+      continue
+    }
+    if (!inHunk && line.startsWith('--- ') && (lines[index + 1] ?? '').startsWith('+++ ')) {
+      indices.add(index)
+      indices.add(index + 1)
+      index += 1
+    }
+  }
+  return indices
+}
+
 function toLines(value: unknown, maxLines: number): { lines: string[]; truncated: boolean } {
   if (typeof value !== 'string') {
     return { lines: [], truncated: false }
@@ -62,23 +95,35 @@ export function diffFromText(
     return null
   }
   const bounded = toLines(text, maxLines)
+  const headers = fileHeaderIndices(bounded.lines)
   let added = 0
   let removed = 0
-  const lines = bounded.lines.map((line): NativeChatDiffLine => {
-    if (line.startsWith('@@') || line.startsWith('diff ') || line.startsWith('index ')) {
+  const lines = bounded.lines.map((line, index): NativeChatDiffLine => {
+    if (
+      headers.has(index) ||
+      line.startsWith('@@') ||
+      line.startsWith('diff ') ||
+      line.startsWith('index ')
+    ) {
       return { kind: 'meta', text: line }
     }
-    if (line.startsWith('+') && !line.startsWith('+++')) {
+    if (line.startsWith('+')) {
       added += 1
       return { kind: 'add', text: line.slice(1) }
     }
-    if (line.startsWith('-') && !line.startsWith('---')) {
+    if (line.startsWith('-')) {
       removed += 1
       return { kind: 'del', text: line.slice(1) }
     }
     return { kind: 'context', text: line }
   })
-  if (added + removed < 2) {
+  // A hunk header or `diff --git` proves this really is diff text, so a
+  // single-line change renders; without one, two markers guard against
+  // colouring prose that merely opens a line with `-`.
+  const isStructuredDiff = bounded.lines.some(
+    (line) => HUNK_HEADER.test(line) || line.startsWith('diff --git ')
+  )
+  if (added + removed < (isStructuredDiff ? 1 : 2)) {
     return null
   }
   return bounded.truncated ? [...lines.slice(0, maxLines - 1), DIFF_TRUNCATED_LINE] : lines