From f9b857463be40304c1745d7f2109c2cd499eb271 Mon Sep 17 00:00:00 2001
From: Calixte Denizet <calixte.denizet@gmail.com>
Date: Wed, 3 Dec 2025 15:51:34 +0100
Subject: [PATCH] Use HTMLElement.scrollIntoView when showing a thumbnail

---
 test/integration/thumbnail_view_spec.mjs | 50 +++++++++++++++++++++++-
 web/pdf_thumbnail_viewer.js              |  9 +++--
 web/viewer.css                           |  1 +
 3 files changed, 56 insertions(+), 4 deletions(-)

diff --git a/test/integration/thumbnail_view_spec.mjs b/test/integration/thumbnail_view_spec.mjs
index 428813604..0dae8eecf 100644
--- a/test/integration/thumbnail_view_spec.mjs
+++ b/test/integration/thumbnail_view_spec.mjs
@@ -1,4 +1,4 @@
-import { closePages, loadAndWait } from "./test_utils.mjs";
+import { awaitPromise, closePages, loadAndWait } from "./test_utils.mjs";
 
 describe("PDF Thumbnail View", () => {
   describe("Works without errors", () => {
@@ -32,4 +32,52 @@ describe("PDF Thumbnail View", () => {
       );
     });
   });
+
+  describe("The view is scrolled correctly", () => {
+    let pages;
+
+    beforeEach(async () => {
+      pages = await loadAndWait("tracemonkey.pdf", "#sidebarToggleButton");
+    });
+
+    afterEach(async () => {
+      await closePages(pages);
+    });
+
+    async function goToPage(page, number) {
+      const handle = await page.evaluateHandle(
+        num => [
+          new Promise(resolve => {
+            const container = document.getElementById("thumbnailView");
+            container.addEventListener("scrollend", resolve, { once: true });
+            // eslint-disable-next-line no-undef
+            PDFViewerApplication.pdfLinkService.goToPage(num);
+          }),
+        ],
+        number
+      );
+      return awaitPromise(handle);
+    }
+
+    it("should scroll the view", async () => {
+      await Promise.all(
+        pages.map(async ([browserName, page]) => {
+          await page.click("#sidebarToggleButton");
+
+          for (const pageNum of [14, 1, 13, 2]) {
+            await goToPage(page, pageNum);
+            const thumbSelector = `.thumbnailImage[data-l10n-args='{"page":${pageNum}}']`;
+            await page.waitForSelector(
+              `.thumbnail:has(${thumbSelector}).selected`,
+              { visible: true }
+            );
+            const src = await page.$eval(thumbSelector, el => el.src);
+            expect(src)
+              .withContext(`In ${browserName}`)
+              .toMatch(/^blob:http:/);
+          }
+        })
+      );
+    });
+  });
 });
diff --git a/web/pdf_thumbnail_viewer.js b/web/pdf_thumbnail_viewer.js
index 2079b641b..46b7192fc 100644
--- a/web/pdf_thumbnail_viewer.js
+++ b/web/pdf_thumbnail_viewer.js
@@ -24,13 +24,16 @@ import {
   getVisibleElements,
   isValidRotation,
   RenderingStates,
-  scrollIntoView,
   watchScroll,
 } from "./ui_utils.js";
 import { PDFThumbnailView } from "./pdf_thumbnail_view.js";
 
-const THUMBNAIL_SCROLL_MARGIN = -19;
 const THUMBNAIL_SELECTED_CLASS = "selected";
+const SCROLL_OPTIONS = {
+  behavior: "instant",
+  container: "nearest",
+  block: "nearest",
+};
 
 /**
  * @typedef {Object} PDFThumbnailViewerOptions
@@ -139,7 +142,7 @@ class PDFThumbnailViewer {
         }
       }
       if (shouldScroll) {
-        scrollIntoView(thumbnailView.div, { top: THUMBNAIL_SCROLL_MARGIN });
+        thumbnailView.div.scrollIntoView(SCROLL_OPTIONS);
       }
     }
 
diff --git a/web/viewer.css b/web/viewer.css
index 896b91a5a..d2ef9366d 100644
--- a/web/viewer.css
+++ b/web/viewer.css
@@ -729,6 +729,7 @@ body {
     height: auto;
 
     > .thumbnail {
+      scroll-margin-block: 19px;
       width: var(--thumbnail-width);
       margin: 0 10px 5px;
       padding: 1px;