perf(diff): defer large diffs until user loads them

Prevents UI freeze when opening files with very large diffs by
deferring render until the user explicitly loads them.

src/renderer/src/components/editor/CombinedDiffFileTree.test.ts

@@ -9,12 +9,20 @@ import {
   COMBINED_DIFF_FILE_TREE_QUERY_MAX_BYTES,
   getCombinedDiffBranchEntriesInTreeOrder,
   getFilteredCombinedDiffFileTreeEntries,
-  isCombinedDiffFileTreeQueryTooLarge
+  isCombinedDiffFileTreeQueryTooLarge,
+  isCombinedDiffSectionViewed
 } from './combined-diff-file-tree-model'
 import type { GitBranchChangeEntry } from '../../../../shared/git-diff-compare-types'
 import type { GitStatusEntry } from '../../../../shared/git-status-types'
 
 describe('CombinedDiffFileTree navigation mapping', () => {
+  it('does not count deferred sections as viewed', () => {
+    expect(isCombinedDiffSectionViewed({ loading: false, loadOnDemand: true })).toBe(false)
+    expect(isCombinedDiffSectionViewed({ loading: false, loadOnDemand: false })).toBe(true)
+    expect(isCombinedDiffSectionViewed({ loading: false })).toBe(true)
+    expect(isCombinedDiffSectionViewed({ loading: true, loadOnDemand: false })).toBe(false)
+  })
+
   it('disambiguates uncommitted entries with the same path by area', () => {
     const staged: GitStatusEntry = { path: 'src/App.tsx', status: 'modified', area: 'staged' }
     const unstaged: GitStatusEntry = { path: 'src/App.tsx', status: 'modified', area: 'unstaged' }

src/renderer/src/components/editor/CombinedDiffFileTree.tsx

@@ -28,7 +28,8 @@ export {
   createCombinedDiffSectionIndexMap,
   getCombinedDiffFileTreeNavigationIndex,
   getCombinedDiffFileTreeSectionKey,
-  handleCombinedDiffFileTreeNavigation
+  handleCombinedDiffFileTreeNavigation,
+  isCombinedDiffSectionViewed
 } from './combined-diff-file-tree-model'
 
 const UNCOMMITTED_AREA_ORDER: readonly GitStagingArea[] = ['unstaged', 'staged', 'untracked']

src/renderer/src/components/editor/CombinedDiffViewer.tsx

@@ -52,7 +52,8 @@ import { DiffNotesSendMenu } from './DiffNotesSendMenu'
 import {
   CombinedDiffFileTree,
   createCombinedDiffSectionIndexMap,
-  handleCombinedDiffFileTreeNavigation
+  handleCombinedDiffFileTreeNavigation,
+  isCombinedDiffSectionViewed
 } from './CombinedDiffFileTree'
 import { getCombinedDiffFileTreeSectionKey } from './combined-diff-file-tree-model'
 import {
@@ -310,6 +311,13 @@ export default function CombinedDiffViewer({
   const deferredLoadRequestsRef = useRef<Set<number>>(new Set())
   const sectionsRef = useRef<DiffSection[]>([])
   const generationRef = useRef(0)
+  // Why: status polling can replace the entries array without changing its
+  // metadata; don't reinitialize and cancel an in-flight explicit diff load.
+  const initializedEntryStateRef = useRef<{
+    viewStateKey: string
+    entrySignature: string
+    hasUncommittedEntriesSnapshot: boolean
+  } | null>(null)
   // Why: per-section reload token, so a sibling's reload can't discard this section's in-flight load.
   const sectionLoadTokensRef = useRef<Map<number, number>>(new Map())
   const renderedIndicesRef = useRef<Set<number>>(new Set())
@@ -520,6 +528,19 @@ export default function CombinedDiffViewer({
 
   // Why: tab/worktree switches unmount this viewer; cache by pane key so remount restores sections+scroll before repaint.
   useLayoutEffect(() => {
+    const initializedEntryState = initializedEntryStateRef.current
+    if (
+      initializedEntryState?.viewStateKey === viewStateKey &&
+      initializedEntryState.entrySignature === entrySignature &&
+      initializedEntryState.hasUncommittedEntriesSnapshot === hasUncommittedEntriesSnapshot
+    ) {
+      return
+    }
+    initializedEntryStateRef.current = {
+      viewStateKey,
+      entrySignature,
+      hasUncommittedEntriesSnapshot
+    }
     deferredLoadRequestsRef.current.clear()
     const cached = combinedDiffViewStateCache.get(viewStateKey)
     const canRestoreSnapshotSectionsByKey =
@@ -1131,7 +1152,12 @@ export default function CombinedDiffViewer({
     setActiveTreeSectionState({ entrySignature, key: null })
   }
   const viewedSectionKeys = React.useMemo(
-    () => new Set(sections.filter((section) => !section.loading).map((section) => section.key)),
+    () =>
+      new Set(
+        sections
+          .filter((section) => isCombinedDiffSectionViewed(section))
+          .map((section) => section.key)
+      ),
     [sections]
   )
   const handleTreeNavigate = useCallback(

src/renderer/src/components/editor/combined-diff-file-tree-model.ts

@@ -1,6 +1,7 @@
 import { basename } from '@/lib/path'
 import type { GitBranchChangeEntry } from '../../../../shared/git-diff-compare-types'
 import type { GitStatusEntry } from '../../../../shared/git-status-types'
+import type { DiffSection } from './diff-section-types'
 import { isClipboardTextByteLengthOverLimit } from '../../../../shared/clipboard-text'
 import {
   buildSourceControlTree,
@@ -38,6 +39,12 @@ export function createCombinedDiffSectionIndexMap(
   return new Map(sections.map((section, index) => [section.key, index]))
 }
 
+export function isCombinedDiffSectionViewed(
+  section: Pick<DiffSection, 'loading' | 'loadOnDemand'>
+): boolean {
+  return !section.loading && section.loadOnDemand !== true
+}
+
 export function getCombinedDiffFileTreeNavigationIndex({
   mode,
   entry,

src/renderer/src/components/editor/combined-diff-on-demand-load.test.ts

@@ -26,4 +26,16 @@ describe('combined diff on-demand loading', () => {
   it('automatically loads diffs when line counts are unavailable', () => {
     expect(shouldLoadCombinedDiffOnDemand({ added: undefined, removed: undefined })).toBe(false)
   })
+
+  it('defers untracked diffs when only additions are reported', () => {
+    expect(shouldLoadCombinedDiffOnDemand({ added: MAX_AUTOMATIC_DIFF_CHANGED_LINES + 1 })).toBe(
+      true
+    )
+  })
+
+  it('defers diffs when only removals are reported', () => {
+    expect(shouldLoadCombinedDiffOnDemand({ removed: MAX_AUTOMATIC_DIFF_CHANGED_LINES + 1 })).toBe(
+      true
+    )
+  })
 })

src/renderer/src/components/editor/combined-diff-on-demand-load.ts

@@ -7,8 +7,10 @@ export function shouldLoadCombinedDiffOnDemand({
   added?: number
   removed?: number
 }): boolean {
-  if (added === undefined || removed === undefined) {
+  // Untracked text files report additions only; both fields are absent when
+  // line counts are unavailable (for example, binary or oversized files).
+  if (added === undefined && removed === undefined) {
     return false
   }
-  return added + removed > MAX_AUTOMATIC_DIFF_CHANGED_LINES
+  return (added ?? 0) + (removed ?? 0) > MAX_AUTOMATIC_DIFF_CHANGED_LINES
 }

src/renderer/src/i18n/locales/en.json

@@ -14869,6 +14869,10 @@
           "directoryAccessRequestMultiple": "This preview wants to read files in {{folders}}.",
           "allowDirectories": "Allow {{count}} folders",
           "editAddressControl": "Edit address"
+        },
+        "LargeDiffLoadPrompt": {
+          "a0af0198aa": "Large diffs are not rendered by default.",
+          "f7fa7a40d0": "Load diff"
         }
       },
       "diff": {

tests/e2e/large-diff-freeze-repro.spec.ts

@@ -87,14 +87,23 @@ test.describe('Large diff freeze repro', () => {
           if (!store) {
             throw new Error('window.__store is not available')
           }
-          const status = await window.api.git.status({ worktreePath: repoPath })
-          store.getState().setGitStatus(wId, status)
-          const entry = status.entries.find(
+          let status = await window.api.git.status({ worktreePath: repoPath })
+          let entry = status.entries.find(
             (candidate) => candidate.path === relativePath && candidate.area === 'unstaged'
           )
+          // Why: the app may still be settling the just-added worktree's first status read.
+          const statusDeadline = performance.now() + 5_000
+          while (!entry && performance.now() < statusDeadline) {
+            await new Promise((resolve) => window.setTimeout(resolve, 100))
+            status = await window.api.git.status({ worktreePath: repoPath })
+            entry = status.entries.find(
+              (candidate) => candidate.path === relativePath && candidate.area === 'unstaged'
+            )
+          }
           if (!entry) {
             throw new Error(`large diff status entry not found: ${relativePath}`)
           }
+          store.getState().setGitStatus(wId, status)
           store.getState().openAllDiffs(wId, repoPath, undefined, 'unstaged', [entry])
         },
         { wId: worktreeId, repoPath: fixture.repoPath, relativePath: fixture.relativePath }