Display a wait cursor when saving

When the document is a bit long to save (for example in modifying a large document)
we must have something indicating that pdf.js is doing something.

web/app.js

@@ -1133,12 +1133,19 @@ const PDFViewerApplication = {
     }
   },
 
-  downloadOrSave(options = {}) {
-    if (this.pdfDocument?.annotationStorage.size > 0) {
-      this.save(options);
-    } else {
-      this.download(options);
-    }
+  async downloadOrSave(options = {}) {
+    // In the Firefox case, this method MUST always trigger a download.
+    // When the user is closing a modified and unsaved document, we display a
+    // prompt asking for saving or not. In case they save, we must wait for
+    // saving to complete before closing the tab.
+    // So in case this function does not trigger a download, we must trigger a
+    // a message and change PdfjsChild.sys.mjs to take it into account.
+    const { classList } = this.appConfig.appContainer;
+    classList.add("wait");
+    await (this.pdfDocument?.annotationStorage.size > 0
+      ? this.save(options)
+      : this.download(options));
+    classList.remove("wait");
   },
 
   /**

web/viewer.css

@@ -223,6 +223,15 @@ body {
 body {
   background-color: var(--body-bg-color);
   scrollbar-color: var(--scrollbar-color) var(--scrollbar-bg-color);
+
+  &.wait::before {
+    content: "";
+    position: fixed;
+    width: 100%;
+    height: 100%;
+    z-index: 100000;
+    cursor: wait;
+  }
 }
 
 .hidden,