avoid unneeded calls to getBoundingClientRect

lib/hooks/hooks-frame.js

@@ -39,6 +39,7 @@ this.hooksFrame = this.hooksFrame || (() => {
 
 	function hook() {
 		const NEW_FONT_FACE_EVENT = "single-file-new-font-face";
+		const LOAD_OBSERVED_ELEMENTS_EVENT = "single-file-load-observed-elements";
 		const FONT_STYLE_PROPERTIES = {
 			family: "font-family",
 			style: "font-style",
@@ -69,8 +70,6 @@ this.hooksFrame = this.hooksFrame || (() => {
 		}));
 
 		if (window.IntersectionObserver) {
-			const LOAD_OBSERVED_ELEMENTS_EVENT = "single-file-load-observed-elements";
-		
 			const IntersectionObserver = window.IntersectionObserver;
 			const observeIntersection = IntersectionObserver.prototype.observe;
 			const unobserveIntersection = IntersectionObserver.prototype.unobserve;
@@ -112,15 +111,16 @@ this.hooksFrame = this.hooksFrame || (() => {
 			}));
 			window.__defineSetter__("IntersectionObserver", () => { });
 			addEventListener(LOAD_OBSERVED_ELEMENTS_EVENT, () => {
+				const docBoundingRect = document.documentElement.getBoundingClientRect();
 				Array.from(observers).forEach(([intersectionObserver, observer]) => {
+					const rootBoundingRect = observer.options.root.getBoundingClientRect();
 					observer.callback(observedElements.get(intersectionObserver).map(target => {
 						const boundingClientRect = target.getBoundingClientRect();
-						const intersectionRect = target.getBoundingClientRect();
 						const isIntersecting = true;
 						const intersectionRatio = 1;
-						const rootBounds = observer.options && observer.options.root ? observer.options.root.getBoundingClientRect() : document.documentElement.getBoundingClientRect();
+						const rootBounds = observer.options && observer.options.root ? rootBoundingRect : docBoundingRect;
 						const time = 0;
-						return { target, intersectionRatio, boundingClientRect, intersectionRect, isIntersecting, rootBounds, time };
+						return { target, intersectionRatio, boundingClientRect, intersectionRect: boundingClientRect, isIntersecting, rootBounds, time };
 					}), intersectionObserver);
 				});
 			}, false);