diff --git a/web/pdf_sidebar_resizer.js b/web/pdf_sidebar_resizer.js
index 384be23f7..71d557ffe 100644
--- a/web/pdf_sidebar_resizer.js
+++ b/web/pdf_sidebar_resizer.js
@@ -13,7 +13,7 @@
  * limitations under the License.
  */
 
-import { NullL10n } from './ui_utils';
+import { clamp, NullL10n } from './ui_utils';
 
 const SIDEBAR_WIDTH_VAR = '--sidebar-width';
 const SIDEBAR_MIN_WIDTH = 200; // pixels
@@ -47,8 +47,10 @@ class PDFSidebarResizer {
     this.eventBus = eventBus;
     this.l10n = l10n;
 
-    if (typeof CSS === 'undefined' || typeof CSS.supports !== 'function' ||
-        !CSS.supports(SIDEBAR_WIDTH_VAR, `calc(-1 * ${SIDEBAR_MIN_WIDTH}px)`)) {
+    if ((typeof PDFJSDev === 'undefined' || !PDFJSDev.test('MOZCENTRAL')) &&
+        (typeof CSS === 'undefined' || typeof CSS.supports !== 'function' ||
+         !CSS.supports(SIDEBAR_WIDTH_VAR,
+                       `calc(-1 * ${SIDEBAR_MIN_WIDTH}px)`))) {
       console.warn('PDFSidebarResizer: ' +
                    'The browser does not support resizing of the sidebar.');
       return;
@@ -82,19 +84,14 @@ class PDFSidebarResizer {
     }
     // Prevent the sidebar from becoming too narrow, or from occupying more
     // than half of the available viewer width.
-    const maxWidth = Math.floor(this.outerContainerWidth / 2);
-    if (width > maxWidth) {
-      width = maxWidth;
-    }
-    if (width < SIDEBAR_MIN_WIDTH) {
-      width = SIDEBAR_MIN_WIDTH;
-    }
+    const newWidth = clamp(width, SIDEBAR_MIN_WIDTH,
+                           Math.floor(this.outerContainerWidth / 2));
     // Only update the UI when the sidebar width did in fact change.
-    if (width === this._width) {
+    if (newWidth === this._width) {
       return false;
     }
-    this._width = width;
-    this.doc.style.setProperty(SIDEBAR_WIDTH_VAR, `${width}px`);
+    this._width = newWidth;
+    this.doc.style.setProperty(SIDEBAR_WIDTH_VAR, `${newWidth}px`);
     return true;
   }
 
@@ -154,30 +151,33 @@ class PDFSidebarResizer {
     this.eventBus.on('resize', (evt) => {
       // When the *entire* viewer is resized, such that it becomes narrower,
       // ensure that the sidebar doesn't end up being too wide.
-      if (evt && evt.source === window) {
-        // Always reset the cached width when the viewer is resized.
-        this._outerContainerWidth = null;
-
-        if (this._width) {
-          // NOTE: If the sidebar is closed, we don't need to worry about
-          //       visual glitches nor ensure that rendering is triggered.
-          if (this.sidebarOpen) {
-            this.outerContainer.classList.add(SIDEBAR_RESIZING_CLASS);
-            let updated = this._updateWidth(this._width);
-
-            Promise.resolve().then(() => {
-              this.outerContainer.classList.remove(SIDEBAR_RESIZING_CLASS);
-              // Trigger rendering if the sidebar width changed, to avoid
-              // depending on the order in which 'resize' events are handled.
-              if (updated) {
-                this.eventBus.dispatch('resize', { source: this, });
-              }
-            });
-          } else {
-            this._updateWidth(this._width);
-          }
-        }
+      if (!evt || evt.source !== window) {
+        return;
       }
+      // Always reset the cached width when the viewer is resized.
+      this._outerContainerWidth = null;
+
+      if (!this._width) {
+        // The sidebar hasn't been resized, hence no need to adjust its width.
+        return;
+      }
+      // NOTE: If the sidebar is closed, we don't need to worry about
+      //       visual glitches nor ensure that rendering is triggered.
+      if (!this.sidebarOpen) {
+        this._updateWidth(this._width);
+        return;
+      }
+      this.outerContainer.classList.add(SIDEBAR_RESIZING_CLASS);
+      let updated = this._updateWidth(this._width);
+
+      Promise.resolve().then(() => {
+        this.outerContainer.classList.remove(SIDEBAR_RESIZING_CLASS);
+        // Trigger rendering if the sidebar width changed, to avoid
+        // depending on the order in which 'resize' events are handled.
+        if (updated) {
+          this.eventBus.dispatch('resize', { source: this, });
+        }
+      });
     });
   }
 }
diff --git a/web/ui_utils.js b/web/ui_utils.js
index e1235759a..cb7108a75 100644
--- a/web/ui_utils.js
+++ b/web/ui_utils.js
@@ -898,6 +898,7 @@ export {
   NullL10n,
   EventBus,
   getGlobalEventBus,
+  clamp,
   ProgressBar,
   getPDFFileNameFromURL,
   noContextMenuHandler,