diff --git a/apps/x/apps/renderer/src/components/code/cm.ts b/apps/x/apps/renderer/src/components/code/cm.ts
index 7fe9d922..4b75c1b2 100644
--- a/apps/x/apps/renderer/src/components/code/cm.ts
+++ b/apps/x/apps/renderer/src/components/code/cm.ts
@@ -53,6 +53,18 @@ export function cmBaseExtensions(isDark: boolean): Extension[] {
           color: isDark ? '#6b7280' : '#9ca3af',
         },
         '&.cm-focused': { outline: 'none' },
+        // GitHub-style expander bar for folded unchanged regions (@codemirror/merge).
+        '.cm-collapsedLines': {
+          backgroundColor: isDark ? 'rgba(56, 139, 253, 0.15)' : 'rgba(9, 105, 218, 0.08)',
+          backgroundImage: 'none',
+          color: isDark ? '#79c0ff' : '#0969da',
+          padding: '4px 12px',
+          fontSize: '11px',
+          cursor: 'pointer',
+        },
+        '.cm-collapsedLines:hover': {
+          backgroundColor: isDark ? 'rgba(56, 139, 253, 0.25)' : 'rgba(9, 105, 218, 0.15)',
+        },
       },
       { dark: isDark },
     ),
diff --git a/apps/x/apps/renderer/src/components/code/diff-viewer.tsx b/apps/x/apps/renderer/src/components/code/diff-viewer.tsx
index 6d34e498..5b6c7b4d 100644
--- a/apps/x/apps/renderer/src/components/code/diff-viewer.tsx
+++ b/apps/x/apps/renderer/src/components/code/diff-viewer.tsx
@@ -1,7 +1,7 @@
 import { useEffect, useRef, useState } from 'react'
 import { MergeView, unifiedMergeView } from '@codemirror/merge'
 import { EditorView } from '@codemirror/view'
-import { Columns2, Rows2, X } from 'lucide-react'
+import { Columns2, FoldVertical, Rows2, UnfoldVertical, X } from 'lucide-react'
 import { useTheme } from '@/contexts/theme-context'
 import { Button } from '@/components/ui/button'
 import { cmBaseExtensions, cmLanguageFor } from './cm'
@@ -22,6 +22,9 @@ export function DiffViewer({
   const isDark = resolvedTheme === 'dark'
   const containerRef = useRef<HTMLDivElement>(null)
   const [mode, setMode] = useState<'split' | 'unified'>('split')
+  // GitHub-style: unchanged regions fold into "⋯ N lines" bars (each clickable
+  // to reveal); "Expand all" rebuilds the view with nothing collapsed.
+  const [collapseUnchanged, setCollapseUnchanged] = useState(true)
   const [diff, setDiff] = useState<{ oldText: string; newText: string; isBinary: boolean; tooLarge: boolean } | null>(null)
   const [error, setError] = useState<string | null>(null)
 
@@ -44,19 +47,27 @@ export function DiffViewer({
     void cmLanguageFor(path).then((language) => {
       if (cancelled || !containerRef.current) return
       const extensions = [...cmBaseExtensions(isDark), ...(language ? [language] : [])]
+      // Same context margins GitHub uses: keep a few lines around each hunk,
+      // only fold stretches long enough to be worth hiding.
+      const collapse = collapseUnchanged ? { margin: 3, minSize: 6 } : undefined
       if (mode === 'split') {
         view = new MergeView({
           a: { doc: diff.oldText, extensions },
           b: { doc: diff.newText, extensions },
           parent,
           gutter: true,
+          ...(collapse ? { collapseUnchanged: collapse } : {}),
         })
       } else {
         view = new EditorView({
           doc: diff.newText,
           extensions: [
             ...extensions,
-            unifiedMergeView({ original: diff.oldText, mergeControls: false }),
+            unifiedMergeView({
+              original: diff.oldText,
+              mergeControls: false,
+              ...(collapse ? { collapseUnchanged: collapse } : {}),
+            }),
           ],
           parent,
         })
@@ -67,12 +78,22 @@ export function DiffViewer({
       cancelled = true
       view?.destroy()
     }
-  }, [diff, mode, isDark, path])
+  }, [diff, mode, isDark, path, collapseUnchanged])
 
   return (
     <div className="flex h-full min-h-0 flex-col">
       <div className="flex items-center gap-2 border-b px-3 py-1.5">
         <span className="min-w-0 flex-1 truncate font-mono text-xs text-foreground/90" title={path}>{path}</span>
+        <Button
+          variant="ghost"
+          size="sm"
+          className="h-7 px-2 text-xs text-muted-foreground"
+          onClick={() => setCollapseUnchanged((c) => !c)}
+          title={collapseUnchanged ? 'Show the whole file' : 'Collapse unchanged regions'}
+        >
+          {collapseUnchanged ? <UnfoldVertical className="size-3.5" /> : <FoldVertical className="size-3.5" />}
+          {collapseUnchanged ? 'Expand all' : 'Collapse'}
+        </Button>
         <Button
           variant="ghost"
           size="sm"