fix(ui): isolate OpenTUI scroll reset compatibility

.changeset/quiet-opentui-scroll-reset.md

@@ -0,0 +1,5 @@
+---
+"hunkdiff": patch
+---
+
+Fail clearly when an OpenTUI upgrade removes the shifted-wheel scroll reset Hunk requires.

src/ui/components/panes/DiffPane.test.tsx

@@ -0,0 +1,19 @@
+import { describe, expect, test } from "bun:test";
+import { ScrollBoxRenderable } from "@opentui/core";
+import { resetOpenTuiScrollAccumulators } from "./DiffPane";
+
+describe("resetOpenTuiScrollAccumulators", () => {
+  test("requires the OpenTUI 0.5.1 compatibility operation", () => {
+    const resetScrollAccumulators = (
+      ScrollBoxRenderable.prototype as unknown as { resetScrollAccumulators?: () => void }
+    ).resetScrollAccumulators;
+
+    expect(resetScrollAccumulators).toBeFunction();
+  });
+
+  test("fails clearly when an OpenTUI upgrade removes the compatibility operation", () => {
+    expect(() => resetOpenTuiScrollAccumulators({} as unknown as ScrollBoxRenderable)).toThrow(
+      "OpenTUI 0.5.1 ScrollBoxRenderable.resetScrollAccumulators is required after shifted wheel input.",
+    );
+  });
+});

src/ui/components/panes/DiffPane.tsx

@@ -113,6 +113,27 @@ import {
 
 const EMPTY_VISIBLE_AGENT_NOTES: VisibleAgentNote[] = [];
 
+/**
+ * Resets OpenTUI's wheel remainder after Hunk reroutes a shifted wheel event.
+ *
+ * OpenTUI 0.5.1 keeps this operation private, so retain this compatibility bridge only until
+ * OpenTUI exposes a public reset API. A missing operation must fail loudly rather than let a
+ * later vertical wheel event consume the stale remainder and move the review viewport.
+ */
+export function resetOpenTuiScrollAccumulators(scrollBox: ScrollBoxRenderable) {
+  const compatibilityScrollBox = scrollBox as unknown as {
+    resetScrollAccumulators?: () => void;
+  };
+
+  if (!compatibilityScrollBox.resetScrollAccumulators) {
+    throw new Error(
+      "OpenTUI 0.5.1 ScrollBoxRenderable.resetScrollAccumulators is required after shifted wheel input. Update this compatibility bridge when upgrading OpenTUI.",
+    );
+  }
+
+  compatibilityScrollBox.resetScrollAccumulators();
+}
+
 /**
  * Clamp one vertical scroll target into the currently reachable review-stream extent.
  *
@@ -479,9 +500,7 @@ export function DiffPane({
 
         currentScrollBox.scrollTo({ x: preservedScrollLeft, y: preservedScrollTop });
         currentScrollBox.scrollAcceleration.reset();
-        (
-          currentScrollBox as unknown as { resetScrollAccumulators?: () => void }
-        ).resetScrollAccumulators?.();
+        resetOpenTuiScrollAccumulators(currentScrollBox);
       });
 
       event.preventDefault();

test/pty/layout.test.ts

@@ -555,6 +555,40 @@ describe("PTY layout", () => {
     }
   });
 
+  test("shifted mouse-wheel input scrolls code horizontally in a real PTY", async () => {
+    const fixture = harness.createLongWrapFilePair();
+    const session = await harness.launchHunk({
+      args: ["diff", fixture.before, fixture.after, "--mode", "split"],
+      cols: 102,
+      rows: 20,
+    });
+
+    try {
+      const initial = await session.waitForText(/View\s+Navigate\s+Agent\s+Help/, {
+        timeout: 15_000,
+      });
+
+      expect(initial).toContain("this is a very long");
+      expect(initial).not.toContain("ge';");
+
+      let shifted = initial;
+      for (let index = 0; index < 96; index += 1) {
+        // SGR button 69 is a wheel-down event with the Shift modifier.
+        session.writeRaw("\x1b[<69;61;11M");
+        await session.waitIdle();
+        shifted = await session.text({ immediate: true });
+        if (shifted.includes("ge';")) {
+          break;
+        }
+      }
+
+      expect(shifted).toContain("ge';");
+      expect(shifted).not.toContain("this is a very long");
+    } finally {
+      session.close();
+    }
+  });
+
   test("wrap toggles reset horizontal code scrolling in a real PTY", async () => {
     const fixture = harness.createLongWrapFilePair();
     const session = await harness.launchHunk({