Fix Shift+wheel vertical scroll leak

CHANGELOG.md

@@ -11,6 +11,7 @@ All notable user-visible changes to Hunk are documented in this file.
 ### Fixed
 
 - Smoothed mouse-wheel review scrolling so small diffs stay precise while sustained wheel gestures still speed up.
+- Fixed Shift+mouse-wheel horizontal scrolling so it no longer leaks a one-line vertical scroll in some terminals.
 
 ## [0.9.4] - 2026-04-14
 

src/ui/AppHost.interactions.test.tsx

@@ -701,6 +701,40 @@ describe("App interactions", () => {
     }
   });
 
+  test("shift plus native horizontal wheel events do not move the vertical review position", async () => {
+    const setup = await testRender(<AppHost bootstrap={createWrapScrollBootstrap()} />, {
+      width: 92,
+      height: 20,
+    });
+
+    try {
+      await flush(setup);
+
+      let frame = setup.captureCharFrame();
+      const initialTopLine = firstVisibleAddedLineNumber(frame);
+      expect(initialTopLine).toBeTruthy();
+      expect(frame).not.toContain("viewport anchoring");
+
+      for (let index = 0; index < 8; index += 1) {
+        await act(async () => {
+          await setup.mockMouse.scroll(60, 10, "right", { modifiers: { shift: true } });
+        });
+        await flush(setup);
+        frame = setup.captureCharFrame();
+        if (frame.includes("viewport anchoring")) {
+          break;
+        }
+      }
+
+      expect(frame).toContain("viewport anchoring");
+      expect(firstVisibleAddedLineNumber(frame)).toBe(initialTopLine);
+    } finally {
+      await act(async () => {
+        setup.renderer.destroy();
+      });
+    }
+  });
+
   test("wrap toggles reset the horizontal code offset", async () => {
     const setup = await testRender(<AppHost bootstrap={createWrapBootstrap()} />, {
       width: 92,

src/ui/components/panes/DiffPane.tsx

@@ -204,6 +204,7 @@ export function DiffPane({
 
       const preservedScrollTop = scrollBox.scrollTop;
       const preservedScrollLeft = scrollBox.scrollLeft;
+      const scrollInfo = event.scroll;
 
       if (direction === "left") {
         onScrollCodeHorizontally(-1);
@@ -217,15 +218,25 @@ export function DiffPane({
         return;
       }
 
-      // OpenTUI runs ScrollBox's own wheel handler after this listener and it does not honor
-      // preventDefault(), so restore the pre-event viewport position on the next microtask.
+      // OpenTUI runs ScrollBox's own wheel handler after this listener and it ignores
+      // preventDefault(). Zero the wheel delta first so native Shift+Wheel left/right events
+      // cannot be remapped back into vertical scroll, then restore the viewport and clear any
+      // residual fractional state on the next microtask as a final guard.
+      if (scrollInfo) {
+        scrollInfo.delta = 0;
+      }
+
       queueMicrotask(() => {
         const currentScrollBox = scrollRef.current;
         if (!currentScrollBox) {
           return;
         }
 
         currentScrollBox.scrollTo({ x: preservedScrollLeft, y: preservedScrollTop });
+        currentScrollBox.scrollAcceleration.reset();
+        (
+          currentScrollBox as unknown as { resetScrollAccumulators?: () => void }
+        ).resetScrollAccumulators?.();
       });
 
       event.preventDefault();