diff --git a/test/integration/reorganize_pages_spec.mjs b/test/integration/reorganize_pages_spec.mjs
index 9a6d060e1..8d7572e77 100644
--- a/test/integration/reorganize_pages_spec.mjs
+++ b/test/integration/reorganize_pages_spec.mjs
@@ -28,24 +28,13 @@ import {
   kbDelete,
   loadAndWait,
   scrollIntoView,
+  showViewsManager,
   waitAndClick,
   waitForDOMMutation,
 } from "./test_utils.mjs";
 
 async function waitForThumbnailVisible(page, pageNums) {
-  const hasAnimations = await page.evaluate(
-    () => !window.matchMedia("(prefers-reduced-motion: reduce)").matches
-  );
-  await page.click("#viewsManagerToggleButton");
-  if (hasAnimations) {
-    await page.waitForSelector("#outerContainer.viewsManagerMoving", {
-      visible: true,
-    });
-  }
-  await page.waitForSelector(
-    "#outerContainer:not(.viewsManagerMoving).viewsManagerOpen",
-    { visible: true }
-  );
+  await showViewsManager(page);
 
   const thumbSelector = "#thumbnailsView .thumbnailImageContainer > img";
   await page.waitForSelector(thumbSelector, { visible: true });
diff --git a/test/integration/test_utils.mjs b/test/integration/test_utils.mjs
index 6e561ac1a..b3f3416c7 100644
--- a/test/integration/test_utils.mjs
+++ b/test/integration/test_utils.mjs
@@ -964,6 +964,26 @@ async function highlightSpan(
   await page.waitForSelector(getEditorSelector(nextId));
 }
 
+async function showViewsManager(page) {
+  const hasAnimations = await page.evaluate(
+    () => !window.matchMedia("(prefers-reduced-motion: reduce)").matches
+  );
+  const movingPromise = hasAnimations
+    ? page.waitForSelector("#outerContainer.viewsManagerMoving", {
+        visible: true,
+      })
+    : Promise.resolve();
+  await page.click("#viewsManagerToggleButton");
+  if (hasAnimations) {
+    await movingPromise;
+  }
+  await page.waitForSelector("#viewsManager", { visible: true });
+  await page.waitForSelector(
+    "#outerContainer:not(.viewsManagerMoving).viewsManagerOpen",
+    { visible: true }
+  );
+}
+
 // Unicode bidi isolation characters, Fluent adds these markers to the text.
 const FSI = "\u2068";
 const PDI = "\u2069";
@@ -1030,6 +1050,7 @@ export {
   selectEditors,
   serializeBitmapDimensions,
   setCaretAt,
+  showViewsManager,
   switchToEditor,
   unselectEditor,
   waitAndClick,
diff --git a/test/integration/thumbnail_view_spec.mjs b/test/integration/thumbnail_view_spec.mjs
index 919431733..28e11fd4d 100644
--- a/test/integration/thumbnail_view_spec.mjs
+++ b/test/integration/thumbnail_view_spec.mjs
@@ -5,6 +5,7 @@ import {
   kbFocusNext,
   loadAndWait,
   PDI,
+  showViewsManager,
 } from "./test_utils.mjs";
 
 function waitForThumbnailVisible(page, pageNum) {
@@ -29,23 +30,6 @@ async function waitForMenu(page, buttonSelector, visible = true) {
   );
 }
 
-async function showViewsManager(page) {
-  const hasAnimations = await page.evaluate(
-    () => !window.matchMedia("(prefers-reduced-motion: reduce)").matches
-  );
-  await page.click("#viewsManagerToggleButton");
-  if (hasAnimations) {
-    await page.waitForSelector("#outerContainer.viewsManagerMoving", {
-      visible: true,
-    });
-  }
-  await page.waitForSelector("#viewsManager", { visible: true });
-  await page.waitForSelector(
-    "#outerContainer:not(.viewsManagerMoving).viewsManagerOpen",
-    { visible: true }
-  );
-}
-
 describe("PDF Thumbnail View", () => {
   describe("Works without errors", () => {
     let pages;
diff --git a/test/integration/viewer_spec.mjs b/test/integration/viewer_spec.mjs
index 68f5e69c7..b9b3f07d1 100644
--- a/test/integration/viewer_spec.mjs
+++ b/test/integration/viewer_spec.mjs
@@ -21,6 +21,7 @@ import {
   getSpanRectFromText,
   loadAndWait,
   scrollIntoView,
+  showViewsManager,
   waitForPageChanging,
   waitForPageRendered,
 } from "./test_utils.mjs";
@@ -1451,6 +1452,82 @@ describe("PDF viewer", () => {
     });
   });
 
+  describe("Outline tree shift-click toggle (PR 20740)", () => {
+    let pages;
+
+    beforeEach(async () => {
+      pages = await loadAndWait(
+        "nested_outline.pdf",
+        "#viewsManagerToggleButton"
+      );
+    });
+
+    afterEach(async () => {
+      await closePages(pages);
+    });
+
+    it("should only toggle the clicked item's subtree, not the whole outline", async () => {
+      await Promise.all(
+        pages.map(async ([browserName, page]) => {
+          // Open the sidebar.
+          await showViewsManager(page);
+
+          // Switch to outline view.
+          await page.click("#viewsManagerSelectorButton");
+          await page.waitForSelector("#outlinesViewMenu", { visible: true });
+          await page.click("#outlinesViewMenu");
+
+          // Wait for the outline tree to render with nesting (toggle buttons).
+          await page.waitForSelector("#outlinesView.withNesting");
+
+          // Initially all three top-level togglers must be expanded.
+          const initialHiddenCount = await page.$$eval(
+            "#outlinesView > .treeItem > .treeItemToggler",
+            togglers =>
+              togglers.filter(t => t.classList.contains("treeItemsHidden"))
+                .length
+          );
+          expect(initialHiddenCount).withContext(`In ${browserName}`).toBe(0);
+
+          // Shift-click the first top-level toggler (section "1. Introduction")
+          // to collapse only its subtree.
+          // The toggler has width/height 0 (visual content via ::before), so
+          // we dispatch the MouseEvent directly rather than using page.click().
+          await page.evaluate(() => {
+            const toggler = document.querySelector(
+              "#outlinesView > .treeItem:nth-child(1) > .treeItemToggler"
+            );
+            toggler.dispatchEvent(
+              new MouseEvent("click", {
+                shiftKey: true,
+                bubbles: true,
+                cancelable: true,
+              })
+            );
+          });
+
+          // Section 1's toggler must now be collapsed.
+          const section1Collapsed = await page.$eval(
+            "#outlinesView > .treeItem:nth-child(1) > .treeItemToggler",
+            t => t.classList.contains("treeItemsHidden")
+          );
+          expect(section1Collapsed).withContext(`In ${browserName}`).toBeTrue();
+
+          // Sections 2 and 3 must remain expanded (the bug collapsed the whole
+          // outline by passing `this.container` instead of
+          // `target.parentNode`).
+          const otherHiddenCount = await page.$$eval(
+            "#outlinesView > .treeItem:nth-child(n+2) > .treeItemToggler",
+            togglers =>
+              togglers.filter(t => t.classList.contains("treeItemsHidden"))
+                .length
+          );
+          expect(otherHiddenCount).withContext(`In ${browserName}`).toBe(0);
+        })
+      );
+    });
+  });
+
   describe("Scroll into view", () => {
     let pages;
 
diff --git a/test/pdfs/.gitignore b/test/pdfs/.gitignore
index 86cd69044..ea075e8b6 100644
--- a/test/pdfs/.gitignore
+++ b/test/pdfs/.gitignore
@@ -874,3 +874,4 @@
 !bug2013793.pdf
 !bug2014080.pdf
 !two_pages.pdf
+!nested_outline.pdf
diff --git a/test/pdfs/nested_outline.pdf b/test/pdfs/nested_outline.pdf
new file mode 100644
index 000000000..fa4a2558e
Binary files /dev/null and b/test/pdfs/nested_outline.pdf differ