fix: address review findings (#2124)

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

@@ -546,6 +546,10 @@ export default function CombinedDiffViewer({
         measuredContentHeight: sectionHeights[index],
         originalContent: section.originalContent,
         modifiedContent: section.modifiedContent,
+        changedLineCount:
+          section.added === undefined && section.removed === undefined
+            ? undefined
+            : (section.added ?? 0) + (section.removed ?? 0),
         useIntrinsicImageHeight: isIntrinsicHeightImageDiff(section.diffResult)
       })
     },

src/renderer/src/components/editor/DiffSectionItem.tsx

@@ -242,6 +242,15 @@ export function DiffSectionItem({
       section.status
     ]
   )
+  const changedLineCount = useMemo(() => {
+    if (lineStats) {
+      return lineStats.added + lineStats.removed
+    }
+    if (section.added === undefined && section.removed === undefined) {
+      return undefined
+    }
+    return (section.added ?? 0) + (section.removed ?? 0)
+  }, [lineStats, section.added, section.removed])
   // Why: image diffs need document-flow height in the combined view; the text
   // fallback only knows line counts and would squash screenshots into one row.
   const useIntrinsicImageHeight = isIntrinsicHeightImageDiff(section.diffResult)
@@ -249,6 +258,7 @@ export function DiffSectionItem({
     measuredContentHeight: sectionHeight,
     originalContent: section.originalContent,
     modifiedContent: section.modifiedContent,
+    changedLineCount,
     useIntrinsicImageHeight
   })
 
@@ -258,6 +268,10 @@ export function DiffSectionItem({
     lineNumberOptionsSubRef.current = applyDiffEditorLineNumberOptions(editor, sideBySide)
     const modified = editor.getModifiedEditor()
 
+    // Why: measuring before Monaco computes hidden unchanged regions records
+    // full-file height, making virtualized combined diffs jump as rows remount.
+    let diffLayoutReady = false
+    let pendingHeightFrame: number | null = null
     const updateHeight = (): void => {
       const contentHeight = editor.getModifiedEditor().getContentHeight()
       setSectionHeights((prev) => {
@@ -267,8 +281,28 @@ export function DiffSectionItem({
         return { ...prev, [index]: contentHeight }
       })
     }
-    modified.onDidContentSizeChange(updateHeight)
-    updateHeight()
+    const requestHeightUpdate = (): void => {
+      if (pendingHeightFrame !== null) {
+        return
+      }
+      pendingHeightFrame = window.requestAnimationFrame(() => {
+        pendingHeightFrame = null
+        updateHeight()
+      })
+    }
+    const markDiffLayoutReady = (): void => {
+      diffLayoutReady = true
+      requestHeightUpdate()
+    }
+    const contentSizeSub = modified.onDidContentSizeChange(() => {
+      if (diffLayoutReady) {
+        requestHeightUpdate()
+      }
+    })
+    const diffUpdateSub = editor.onDidUpdateDiff(markDiffLayoutReady)
+    if (editor.getLineChanges() !== null) {
+      markDiffLayoutReady()
+    }
 
     setModifiedEditor(modified)
     // Why: Monaco disposes inner editors when the DiffEditor container is
@@ -277,6 +311,12 @@ export function DiffSectionItem({
     // methods on a disposed editor instance, and avoids `popover` pointing
     // at a line in an editor that no longer exists.
     modified.onDidDispose(() => {
+      contentSizeSub.dispose()
+      diffUpdateSub.dispose()
+      if (pendingHeightFrame !== null) {
+        window.cancelAnimationFrame(pendingHeightFrame)
+        pendingHeightFrame = null
+      }
       lineNumberOptionsSubRef.current?.dispose()
       lineNumberOptionsSubRef.current = null
       diffEditorRef.current = null

src/renderer/src/components/editor/diff-section-layout.test.ts

@@ -29,6 +29,37 @@ describe('diff section layout', () => {
     ).toBe(76)
   })
 
+  it('uses changed-line count before Monaco reports collapsed diff height', () => {
+    const largeUnchangedFile = Array.from({ length: 10_000 }, (_, index) => `line ${index}`).join(
+      '\n'
+    )
+
+    expect(
+      getDiffSectionBodyHeight({
+        measuredContentHeight: undefined,
+        originalContent: largeUnchangedFile,
+        modifiedContent: `${largeUnchangedFile}\nchanged`,
+        changedLineCount: 1,
+        useIntrinsicImageHeight: false
+      })
+    ).toBe(266)
+  })
+
+  it('caps unmeasured text diffs without changed-line stats', () => {
+    const largeUnchangedFile = Array.from({ length: 10_000 }, (_, index) => `line ${index}`).join(
+      '\n'
+    )
+
+    expect(
+      getDiffSectionBodyHeight({
+        measuredContentHeight: undefined,
+        originalContent: largeUnchangedFile,
+        modifiedContent: `${largeUnchangedFile}\nchanged`,
+        useIntrinsicImageHeight: false
+      })
+    ).toBe(1539)
+  })
+
   it('keeps empty text sections visible', () => {
     expect(
       getDiffSectionBodyHeight({
@@ -93,11 +124,29 @@ describe('diff section layout', () => {
         measuredContentHeight: undefined,
         originalContent: 'one',
         modifiedContent: 'one\ntwo\nthree',
+        changedLineCount: 2,
         useIntrinsicImageHeight: false
       })
     ).toBe(104)
   })
 
+  it('uses changed-line count for large virtualized expanded sections', () => {
+    const largeUnchangedFile = Array.from({ length: 10_000 }, (_, index) => `line ${index}`).join(
+      '\n'
+    )
+
+    expect(
+      getDiffSectionEstimatedHeight({
+        collapsed: false,
+        measuredContentHeight: undefined,
+        originalContent: largeUnchangedFile,
+        modifiedContent: `${largeUnchangedFile}\nchanged`,
+        changedLineCount: 1,
+        useIntrinsicImageHeight: false
+      })
+    ).toBe(294)
+  })
+
   it('estimates collapsed virtualized sections as header-only rows', () => {
     expect(
       getDiffSectionEstimatedHeight({

src/renderer/src/components/editor/diff-section-layout.ts

@@ -4,11 +4,14 @@ const DIFF_LINE_HEIGHT = 19
 const DIFF_SECTION_PADDING_HEIGHT = 19
 const MIN_DIFF_SECTION_BODY_HEIGHT = 60
 const DIFF_SECTION_HEADER_HEIGHT = 28
+const DIFF_UNCHANGED_CONTEXT_LINE_ESTIMATE = 12
+const MAX_UNMEASURED_TEXT_BODY_LINES = 80
 
 type DiffSectionBodyHeightInput = {
   measuredContentHeight: number | undefined
   originalContent: string
   modifiedContent: string
+  changedLineCount?: number
   useIntrinsicImageHeight: boolean
 }
 
@@ -20,6 +23,7 @@ export function getDiffSectionBodyHeight({
   measuredContentHeight,
   originalContent,
   modifiedContent,
+  changedLineCount,
   useIntrinsicImageHeight
 }: DiffSectionBodyHeightInput): number | undefined {
   if (useIntrinsicImageHeight) {
@@ -30,11 +34,24 @@ export function getDiffSectionBodyHeight({
     return measuredContentHeight + DIFF_SECTION_PADDING_HEIGHT
   }
 
+  const fullLineCount = Math.max(
+    originalContent.split('\n').length,
+    modifiedContent.split('\n').length
+  )
+  const estimatedLineCount =
+    changedLineCount !== undefined
+      ? Math.min(
+          fullLineCount,
+          Math.max(2, changedLineCount + DIFF_UNCHANGED_CONTEXT_LINE_ESTIMATE)
+        )
+      : Math.min(fullLineCount, MAX_UNMEASURED_TEXT_BODY_LINES)
+
+  // Why: combined diffs hide unchanged regions inside Monaco. Before Monaco
+  // reports its collapsed content height, sizing from full file length makes
+  // large files flash open and forces the virtualizer to jump on scroll.
   return Math.max(
     MIN_DIFF_SECTION_BODY_HEIGHT,
-    Math.max(originalContent.split('\n').length, modifiedContent.split('\n').length) *
-      DIFF_LINE_HEIGHT +
-      DIFF_SECTION_PADDING_HEIGHT
+    estimatedLineCount * DIFF_LINE_HEIGHT + DIFF_SECTION_PADDING_HEIGHT
   )
 }
 
@@ -43,6 +60,7 @@ export function getDiffSectionEstimatedHeight({
   measuredContentHeight,
   originalContent,
   modifiedContent,
+  changedLineCount,
   useIntrinsicImageHeight
 }: DiffSectionBodyHeightInput & { collapsed: boolean }): number {
   if (collapsed) {
@@ -55,6 +73,7 @@ export function getDiffSectionEstimatedHeight({
       measuredContentHeight,
       originalContent,
       modifiedContent,
+      changedLineCount,
       useIntrinsicImageHeight
     }) ?? MIN_DIFF_SECTION_BODY_HEIGHT)
   )