fix browserless throwing when ResizeObserver not found (visual regression tests etc)

src/Display/WebDisplayInteractionManager.ts

@@ -103,7 +103,7 @@ export class WebDisplayInteractionManager extends AbstractDisplayInteractionMana
 
         const entries: HTMLElement[] = [];
         const self: WebDisplayInteractionManager = this;
-        if(ResizeObserver){
+        if(window.ResizeObserver){ // if(ResizeObserver) throws if ResizeObserver not found (browserless)
             this.resizeObserver = new ResizeObserver((observedElements: ResizeObserverEntry[]) => {
                 WebDisplayInteractionManager.resizeCallback(observedElements, self);
             });
@@ -111,7 +111,7 @@ export class WebDisplayInteractionManager extends AbstractDisplayInteractionMana
         while (nextOffsetParent) {
             this.fullOffsetLeft += nextOffsetParent.offsetLeft;
             this.fullOffsetTop += nextOffsetParent.offsetTop;
-            if(!ResizeObserver){
+            if(!this.resizeObserver){
                 entries.push(nextOffsetParent);
             } else {
                 this.resizeObserver.observe(nextOffsetParent);
@@ -119,7 +119,7 @@ export class WebDisplayInteractionManager extends AbstractDisplayInteractionMana
             nextOffsetParent = nextOffsetParent.offsetParent as HTMLElement;
         }
 
-        if(!ResizeObserver){
+        if(!this.resizeObserver){
             let resizeListener: (this: Window, ev: UIEvent) => any = (): void => {
                 WebDisplayInteractionManager.resizeCallback(entries, self);
             };