Ctrl and the wheel over a PDF

The same gesture Aether has, whose comment already claimed InkNote had it.
Registered by hand rather than through onWheel, because React attaches wheel
listeners passively and a passive listener may not call preventDefault —
without which the browser zooms the whole application instead of the
document.

What stays still depends on how the zoom was asked for, and the two answers
differ: the wheel holds the point under the pointer, the buttons hold the
page being read. The buttons held nothing at all before, so three presses of
+ left you somewhere else in the file; that came out of building this and is
fixed with it.

The checks are the step, that the default really was prevented, and that a
point measured on the page is still under the pointer afterwards.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

e2e/pdf.mjs

@@ -270,6 +270,84 @@ try {
   await p.getByTitle("Zoom out").click();
   await p.waitForTimeout(1500);
 
+  /* ── Ctrl and the wheel ──
+   * The claim is not only "the number changes". It is that the browser is kept
+   * out of it — an unprevented Ctrl+wheel zooms the whole application, toolbar
+   * and all — and that the spot under the pointer is the spot that stays put,
+   * which is the whole difference between a viewer that feels steady and one
+   * that jumps to the top of the page at every step. */
+  const zoomNow = () => p.locator("[data-pdf-zoom]").evaluate((el) => Number(el.dataset.pdfZoom));
+  const pageRect = () =>
+    p.locator('[data-pdf-page="1"]').evaluate((el) => {
+      const r = el.getBoundingClientRect();
+      return { top: r.top, height: r.height };
+    });
+
+  const firstPage = await p.locator('[data-pdf-page="1"]').boundingBox();
+  // A point well inside the page, and low enough that holding it still means
+  // scrolling rather than doing nothing.
+  const at = { x: firstPage.x + firstPage.width * 0.5, y: firstPage.y + firstPage.height * 0.6 };
+  const beforeRect = await pageRect();
+  const fraction = (at.y - beforeRect.top) / beforeRect.height;
+  const zoomBefore = await zoomNow();
+
+  const wheeled = await p.evaluate(
+    ([x, y]) => {
+      const el = document.elementFromPoint(x, y);
+      let prevented = null;
+      const spy = (e) => (prevented = e.defaultPrevented);
+      // Added after the viewer's own listener, so it runs after it and can see
+      // whether the default was called off.
+      document.addEventListener("wheel", spy, { passive: true });
+      el.dispatchEvent(
+        new WheelEvent("wheel", { deltaY: -120, ctrlKey: true, bubbles: true, cancelable: true, clientX: x, clientY: y })
+      );
+      document.removeEventListener("wheel", spy);
+      return prevented;
+    },
+    [at.x, at.y]
+  );
+  await p.waitForTimeout(2000);
+
+  check((await zoomNow()) > zoomBefore, "Ctrl and the wheel zoom in", `${zoomBefore} → ${await zoomNow()}`);
+  check(wheeled === true, "and the browser is stopped from zooming itself instead", `defaultPrevented ${wheeled}`);
+
+  const afterRect = await pageRect();
+  const held = afterRect.top + fraction * afterRect.height;
+  check(
+    Math.abs(held - at.y) < 12,
+    "and the spot under the pointer stays under the pointer",
+    `${Math.round(at.y)} → ${Math.round(held)}`
+  );
+
+  await p.evaluate(
+    ([x, y]) => {
+      document
+        .elementFromPoint(x, y)
+        .dispatchEvent(
+          new WheelEvent("wheel", { deltaY: 120, ctrlKey: true, bubbles: true, cancelable: true, clientX: x, clientY: y })
+        );
+    },
+    [at.x, at.y]
+  );
+  await p.waitForTimeout(1800);
+  check((await zoomNow()) === zoomBefore, "and the other way out again", `${await zoomNow()}`);
+
+  /* The buttons hold something else still: the page being read. "Show me this
+     page larger" should not answer by moving to another one. */
+  await p.locator('[data-pdf-page="2"]').scrollIntoViewIfNeeded();
+  await p.waitForTimeout(1200);
+  const readingBefore = await p.locator("[data-pdf-position]").innerText();
+  await p.getByTitle("Zoom in").click();
+  await p.waitForTimeout(2200);
+  check(
+    (await p.locator("[data-pdf-position]").innerText()) === readingBefore,
+    "zooming from the buttons keeps the page being read",
+    `${readingBefore} → ${await p.locator("[data-pdf-position]").innerText()}`
+  );
+  await p.getByTitle("Zoom out").click();
+  await p.waitForTimeout(1500);
+
   /* ── hiding it ── */
   await p.locator("[data-pdf-hide-ink]").click();
   await p.waitForTimeout(600);