Only show the loadingIcon-spinner on visible pages (issue 14242)

This patch preserves the old behaviour of appending a `loadingIcon`-div to all pages that are not yet loaded/rendered. However, the actual `loadingIcon`-spinner (i.e. the `loading-icon.gif` image) will only be displayed on *visible* pages to improve performance.

To avoid having to iterate through all pages in the document, which doesn't seem like a good idea for a PDF document with thousands of pages, we use a combination of the currently visible *and* cached pages to toggle the `loadingIcon`-spinner.

web/base_viewer.js

@@ -158,6 +158,10 @@ class PDFPageViewBuffer {
     return this.#buf.has(view);
   }
 
+  [Symbol.iterator]() {
+    return this.#buf.keys();
+  }
+
   #destroyFirstView() {
     const firstView = this.#buf.keys().next().value;
 
@@ -1402,6 +1406,23 @@ class BaseViewer {
     return this.scroll.down;
   }
 
+  /**
+   * Only show the `loadingIcon`-spinner on visible pages (see issue 14242).
+   */
+  #toggleLoadingIconSpinner(visibleIds) {
+    for (const id of visibleIds) {
+      const pageView = this._pages[id - 1];
+      pageView?.toggleLoadingIconSpinner(/* viewVisible = */ true);
+    }
+    for (const pageView of this.#buffer) {
+      if (visibleIds.has(pageView.id)) {
+        // Handled above, since the "buffer" may not contain all visible pages.
+        continue;
+      }
+      pageView.toggleLoadingIconSpinner(/* viewVisible = */ false);
+    }
+  }
+
   forceRendering(currentlyVisiblePages) {
     const visiblePages = currentlyVisiblePages || this._getVisiblePages();
     const scrollAhead = this.#getScrollAhead(visiblePages);
@@ -1415,6 +1436,8 @@ class BaseViewer {
       scrollAhead,
       preRenderExtra
     );
+    this.#toggleLoadingIconSpinner(visiblePages.ids);
+
     if (pageView) {
       this._ensurePdfPageLoaded(pageView).then(() => {
         this.renderingQueue.renderView(pageView);

web/pdf_page_view.js

@@ -289,7 +289,10 @@ class PDFPageView {
     }
 
     this.loadingIconDiv = document.createElement("div");
-    this.loadingIconDiv.className = "loadingIcon";
+    this.loadingIconDiv.className = "loadingIcon notVisible";
+    if (this._isStandalone) {
+      this.toggleLoadingIconSpinner(/* viewVisible = */ true);
+    }
     this.loadingIconDiv.setAttribute("role", "img");
     this.l10n.get("loading").then(msg => {
       this.loadingIconDiv?.setAttribute("aria-label", msg);
@@ -523,6 +526,13 @@ class PDFPageView {
     return this.viewport.convertToPdfPoint(x, y);
   }
 
+  /**
+   * @ignore
+   */
+  toggleLoadingIconSpinner(viewVisible = false) {
+    this.loadingIconDiv?.classList.toggle("notVisible", !viewVisible);
+  }
+
   draw() {
     if (this.renderingState !== RenderingStates.INITIAL) {
       console.error("Must be in new state before drawing");

web/pdf_viewer.css

@@ -129,6 +129,9 @@
   bottom: 0;
   background: url("images/loading-icon.gif") center no-repeat;
 }
+.pdfViewer .page .loadingIcon.notVisible {
+  background: none;
+}
 
 .pdfPresentationMode .pdfViewer {
   margin-left: 0;