fix(editor): skip shift-wheel capture when a diff pane cannot scroll sideways

Word-wrapped panes never overflow horizontally, so consuming the gesture
left it dead instead of reaching the outer combined-diff list.

src/renderer/src/components/editor/diff-editor-shift-wheel-scroll.test.ts

@@ -9,9 +9,11 @@ type PaneFixture = {
   setScrollLeft: ReturnType<typeof vi.fn<(value: number) => void>>
   getScrollLeft: () => number
   getContainerDomNode: () => HTMLElement
+  getScrollWidth: () => number
+  getLayoutInfo: () => { contentWidth: number }
 }
 
-function createPaneFixture(initialScrollLeft = 10): PaneFixture {
+function createPaneFixture(initialScrollLeft = 10, scrollWidth = 1000): PaneFixture {
   const container = document.createElement('div')
   const input = document.createElement('div')
   let scrollLeft = initialScrollLeft
@@ -26,7 +28,9 @@ function createPaneFixture(initialScrollLeft = 10): PaneFixture {
     input,
     setScrollLeft,
     getScrollLeft: () => scrollLeft,
-    getContainerDomNode: () => container
+    getContainerDomNode: () => container,
+    getScrollWidth: () => scrollWidth,
+    getLayoutInfo: () => ({ contentWidth: 200 })
   }
 }
 
@@ -93,7 +97,26 @@ describe('installDiffEditorShiftWheelScroll', () => {
     dispose()
   })
 
-  it('scrolls the modified pane independently', () => {
+  it('leaves shift input alone when the pane has no horizontal overflow', () => {
+    const original = createPaneFixture(0, 200)
+    const modified = createPaneFixture(0, 200)
+    const onDownstreamWheel = vi.fn()
+    original.input.addEventListener('wheel', onDownstreamWheel)
+    const dispose = installDiffEditorShiftWheelScroll({
+      getOriginalEditor: () => original,
+      getModifiedEditor: () => modified
+    })
+
+    const event = dispatchWheel(original.input, { deltaY: 24, shiftKey: true })
+
+    expect(event.defaultPrevented).toBe(false)
+    expect(original.setScrollLeft).not.toHaveBeenCalled()
+    expect(onDownstreamWheel).toHaveBeenCalledTimes(1)
+    dispose()
+  })
+
+  // Monaco syncs pane scroll itself; this covers listener routing, not product-level pane independence.
+  it('routes the wheel event to the pane under the pointer', () => {
     const original = createPaneFixture()
     const modified = createPaneFixture()
     const dispose = installDiffEditorShiftWheelScroll({

src/renderer/src/components/editor/diff-editor-shift-wheel-scroll.ts

@@ -4,8 +4,8 @@ const WHEEL_LINE_PIXELS = 16
 
 type HorizontalScrollEditor = Pick<
   editor.ICodeEditor,
-  'getContainerDomNode' | 'getScrollLeft' | 'setScrollLeft'
->
+  'getContainerDomNode' | 'getScrollLeft' | 'setScrollLeft' | 'getScrollWidth'
+> & { getLayoutInfo: () => Pick<editor.EditorLayoutInfo, 'contentWidth'> }
 
 type DiffEditorWithPanes = {
   getModifiedEditor: () => HorizontalScrollEditor
@@ -23,6 +23,10 @@ function getHorizontalWheelPixels(event: WheelEvent, pageWidth: number): number
   return delta
 }
 
+function canScrollHorizontally(editor: HorizontalScrollEditor): boolean {
+  return editor.getScrollWidth() > editor.getLayoutInfo().contentWidth
+}
+
 function installPaneShiftWheelScroll(editor: HorizontalScrollEditor): () => void {
   const container = editor.getContainerDomNode()
   const handleWheel = (event: WheelEvent): void => {
@@ -30,6 +34,11 @@ function installPaneShiftWheelScroll(editor: HorizontalScrollEditor): () => void
       return
     }
 
+    // Why: a word-wrapped pane never overflows sideways, so leave the gesture to the outer list.
+    if (!canScrollHorizontally(editor)) {
+      return
+    }
+
     const delta = getHorizontalWheelPixels(event, container.clientWidth)
     if (delta === 0) {
       return