diff --git a/src/components/Output/custom-els/TwoUp/index.ts b/src/components/Output/custom-els/TwoUp/index.ts
index f07d5e6c..4d61d60e 100644
--- a/src/components/Output/custom-els/TwoUp/index.ts
+++ b/src/components/Output/custom-els/TwoUp/index.ts
@@ -42,7 +42,7 @@ export default class TwoUp extends HTMLElement {
       .observe(this, { childList: true });
 
     // Watch for element size changes.
-    if (window.hasOwnProperty('ResizeObserver')) {
+    if ('ResizeObserver' in window) {
       new ResizeObserver(() => this._resetPosition())
         .observe(this);
     } else {
diff --git a/src/components/Output/custom-els/TwoUp/missing-types.d.ts b/src/components/Output/custom-els/TwoUp/missing-types.d.ts
index d2f9c994..b3c129d5 100644
--- a/src/components/Output/custom-els/TwoUp/missing-types.d.ts
+++ b/src/components/Output/custom-els/TwoUp/missing-types.d.ts
@@ -31,21 +31,19 @@ interface DOMRectReadOnly {
   readonly left: number;
 }
 
-declare global {
-  interface ResizeObserverCallback {
-    (entries: ResizeObserverEntry[], observer: ResizeObserver): void;
-  }
+interface ResizeObserverCallback {
+  (entries: ResizeObserverEntry[], observer: ResizeObserver): void;
+}
 
-  interface ResizeObserverEntry {
-    readonly target: Element;
-    readonly contentRect: DOMRectReadOnly;
-  }
+interface ResizeObserverEntry {
+  readonly target: Element;
+  readonly contentRect: DOMRectReadOnly;
+}
 
-  interface ResizeObserver {
-    observe(target: Element): void;
-    unobserve(target: Element): void;
-    disconnect(): void;
-  }
+interface ResizeObserver {
+  observe(target: Element): void;
+  unobserve(target: Element): void;
+  disconnect(): void;
 }
 
 declare var ResizeObserver: {