Fix thumbnails view lazy rendering

Fixes zotero/zotero#4643

src/common/components/sidebar/thumbnails-view.js

@@ -53,7 +53,7 @@ function ThumbnailsView(props) {
 
 	useEffect (() => {
 		let options = {
-			root: containerRef.current,
+			root: document.querySelector('.sidebar-content'),
 			rootMargin: "200px",
 			threshold: 1.0
 		};

src/pdf/pdf-thumbnails.js

@@ -221,6 +221,12 @@ class PDFThumbnails {
 	}
 
 	async render(pageIndexes = [], rerenderOnly) {
+		// If there are no thumbnails being rerendered due to annotation changes,
+		// clear the rendering queue to only render the thumbnails that were recently
+		// scrolled into view in the sidebar thumbnails view
+		if (!this._thumbnails.some(x => x.forceRerender)) {
+			this._renderQueue.end();
+		}
 		for (let pageIndex of pageIndexes) {
 			let thumbnail = this._thumbnails[pageIndex];
 			// Only already rendered thumbnails will be re-rendered, which means