diff --git a/src/core/core_utils.js b/src/core/core_utils.js
index 5980a51f2a7b9..85aedf03b1464 100644
--- a/src/core/core_utils.js
+++ b/src/core/core_utils.js
@@ -669,12 +669,7 @@ function getNewAnnotationsMap(annotationStorage) {
     if (!key.startsWith(AnnotationEditorPrefix)) {
       continue;
     }
-    let annotations = newAnnotationsByPage.get(value.pageIndex);
-    if (!annotations) {
-      annotations = [];
-      newAnnotationsByPage.set(value.pageIndex, annotations);
-    }
-    annotations.push(value);
+    newAnnotationsByPage.getOrInsert(value.pageIndex, []).push(value);
   }
   return newAnnotationsByPage.size > 0 ? newAnnotationsByPage : null;
 }
diff --git a/src/core/document.js b/src/core/document.js
index 9d65d04ac84ab..8c1696d8b0a84 100644
--- a/src/core/document.js
+++ b/src/core/document.js
@@ -1892,10 +1892,7 @@ class PDFDocument {
       orphanFields.put(fieldRef, parentRef);
     }
 
-    if (!promises.has(name)) {
-      promises.set(name, []);
-    }
-    promises.get(name).push(
+    promises.getOrInsert(name, []).push(
       AnnotationFactory.create(
         xref,
         fieldRef,
diff --git a/src/core/struct_tree.js b/src/core/struct_tree.js
index cdd12dfa85d7f..437d35b9a9259 100644
--- a/src/core/struct_tree.js
+++ b/src/core/struct_tree.js
@@ -450,12 +450,7 @@ class StructTreeRoot {
     for (const element of elements) {
       if (element.structTreeParentId) {
         const id = parseInt(element.structTreeParentId.split("_mc")[1], 10);
-        let elems = idToElements.get(id);
-        if (!elems) {
-          elems = [];
-          idToElements.set(id, elems);
-        }
-        elems.push(element);
+        idToElements.getOrInsert(id, []).push(element);
       }
     }
 
diff --git a/src/display/canvas_dependency_tracker.js b/src/display/canvas_dependency_tracker.js
index 0d391c40655de..40c93417ba7fd 100644
--- a/src/display/canvas_dependency_tracker.js
+++ b/src/display/canvas_dependency_tracker.js
@@ -1,3 +1,18 @@
+/* Copyright 2025 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
 import { Util } from "../shared/util.js";
 
 const FORCED_DEPENDENCY_LABEL = "__forcedDependency";
@@ -50,17 +65,11 @@ class BBoxReader {
   }
 }
 
-const ensureDebugMetadata = (map, key) => {
-  if (!map) {
-    return undefined;
-  }
-  let value = map.get(key);
-  if (!value) {
-    value = { dependencies: new Set(), isRenderingOperation: false };
-    map.set(key, value);
-  }
-  return value;
-};
+const ensureDebugMetadata = (map, key) =>
+  map?.getOrInsertComputed(key, () => ({
+    dependencies: new Set(),
+    isRenderingOperation: false,
+  }));
 
 /**
  * @typedef {"lineWidth" | "lineCap" | "lineJoin" | "miterLimit" | "dash" |
diff --git a/src/scripting_api/app.js b/src/scripting_api/app.js
index 3c8619615d421..1680686bc5759 100644
--- a/src/scripting_api/app.js
+++ b/src/scripting_api/app.js
@@ -52,18 +52,14 @@ class App extends PDFObject {
     );
 
     this._timeoutIds = new WeakMap();
-    if (typeof FinalizationRegistry !== "undefined") {
-      // About setTimeOut/setInterval return values (specs):
-      //   The return value of this method must be held in a
-      //   JavaScript variable.
-      //   Otherwise, the timeout object is subject to garbage-collection,
-      //   which would cause the clock to stop.
-      this._timeoutIdsRegistry = new FinalizationRegistry(
-        this._cleanTimeout.bind(this)
-      );
-    } else {
-      this._timeoutIdsRegistry = null;
-    }
+    // About setTimeOut/setInterval return values (specs):
+    //   The return value of this method must be held in a
+    //   JavaScript variable.
+    //   Otherwise, the timeout object is subject to garbage-collection,
+    //   which would cause the clock to stop.
+    this._timeoutIdsRegistry = new FinalizationRegistry(
+      this._cleanTimeout.bind(this)
+    );
 
     this._timeoutCallbackIds = new Map();
     this._timeoutCallbackId = USERACTIVATION_CALLBACKID + 1;
@@ -113,12 +109,12 @@ class App extends PDFObject {
     const timeout = Object.create(null);
     const id = { callbackId, interval };
     this._timeoutIds.set(timeout, id);
-    this._timeoutIdsRegistry?.register(timeout, id);
+    this._timeoutIdsRegistry.register(timeout, id);
     return timeout;
   }
 
   _unregisterTimeout(timeout) {
-    this._timeoutIdsRegistry?.unregister(timeout);
+    this._timeoutIdsRegistry.unregister(timeout);
 
     const data = this._timeoutIds.get(timeout);
     if (!data) {
diff --git a/test/webserver.mjs b/test/webserver.mjs
index 70b270ee01caa..ac36f1451456e 100644
--- a/test/webserver.mjs
+++ b/test/webserver.mjs
@@ -306,7 +306,15 @@ class WebServer {
 
         // Transform with Babel and istanbul plugin
         const result = babel.transformSync(content, {
-          filename: fileURL.pathname,
+          // On Windows, the file URL starts with a slash (e.g.
+          // /C:/path/to/file.js).
+          // This leading slash makes the file path invalid and causes the
+          // instrumentation to fail, so we need to remove it before passing the
+          // path.
+          filename:
+            process.platform === "win32"
+              ? fileURL.pathname.substring(1)
+              : fileURL.pathname,
           plugins: ["babel-plugin-istanbul"],
           sourceMaps: false,
         });
diff --git a/web/pdf_viewer.js b/web/pdf_viewer.js
index c22ede2c2c025..a3b28220a0620 100644
--- a/web/pdf_viewer.js
+++ b/web/pdf_viewer.js
@@ -2270,10 +2270,7 @@ class PDFViewer {
     this.update();
   }
 
-  /**
-   * @private
-   */
-  _getPageAdvance(currentPageNumber, previous = false) {
+  #getPageAdvance(currentPageNumber, previous = false) {
     switch (this._scrollMode) {
       case ScrollMode.WRAPPED: {
         const { views } = this._getVisiblePages(),
@@ -2284,11 +2281,7 @@ class PDFViewer {
           if (percent === 0 || widthPercent < 100) {
             continue;
           }
-          let yArray = pageLayout.get(y);
-          if (!yArray) {
-            pageLayout.set(y, (yArray ||= []));
-          }
-          yArray.push(id);
+          pageLayout.getOrInsert(y, []).push(id);
         }
         // Find the row of the current page.
         for (const yArray of pageLayout.values()) {
@@ -2379,7 +2372,7 @@ class PDFViewer {
       return false;
     }
     const advance =
-      this._getPageAdvance(currentPageNumber, /* previous = */ false) || 1;
+      this.#getPageAdvance(currentPageNumber, /* previous = */ false) || 1;
 
     this.currentPageNumber = Math.min(currentPageNumber + advance, pagesCount);
     return true;
@@ -2396,7 +2389,7 @@ class PDFViewer {
       return false;
     }
     const advance =
-      this._getPageAdvance(currentPageNumber, /* previous = */ true) || 1;
+      this.#getPageAdvance(currentPageNumber, /* previous = */ true) || 1;
 
     this.currentPageNumber = Math.max(currentPageNumber - advance, 1);
     return true;