Add fallback for `PDFViewerApplication.appConfig` for older PDF.js releases

Specify the type of `window.PDFViewerApplication` in src/types/pdfjs.js
add various missing properties required by `PDFIntegration` as well as
adding checks for potentially null/undefined values that were missing
previously.

 - Add a fallback for older versions of PDF.js where `PDFViewerApplication`
   did not have an `appConfig` property [1]
 - Add a missing check for `getPageView` returning `null`

Fixes https://github.com/hypothesis/product-backlog/issues/1205

[1] The property was introduced in https://github.com/mozilla/pdf.js/commit/b6c74f20561b04573cd00dd924484be7f80a09ce

src/annotator/integrations/pdf.js

@@ -23,6 +23,7 @@ import { PDFMetadata } from './pdf-metadata';
  * @typedef {import('../../types/annotator').Integration} Integration
  * @typedef {import('../../types/annotator').SidebarLayout} SidebarLayout
  * @typedef {import('../../types/api').Selector} Selector
+ * @typedef {import('../../types/pdfjs').PDFViewerApplication} PDFViewerApplication
  */
 
 // The viewport and controls for PDF.js start breaking down below about 670px
@@ -61,14 +62,18 @@ export class PDFIntegration {
     this.annotator = annotator;
 
     const window_ = /** @type {HypothesisWindow} */ (window);
-    this.pdfViewer = window_.PDFViewerApplication.pdfViewer;
+
+    // Assume this class is only used if we're in the PDF.js viewer.
+    const pdfViewerApp = /** @type {PDFViewerApplication} */ (window_.PDFViewerApplication);
+
+    this.pdfViewer = pdfViewerApp.pdfViewer;
     this.pdfViewer.viewer.classList.add('has-transparent-text-layer');
 
     // Get the element that contains all of the PDF.js UI. This is typically
     // `document.body`.
-    this.pdfContainer = window_.PDFViewerApplication?.appConfig?.appContainer;
+    this.pdfContainer = pdfViewerApp.appConfig?.appContainer ?? document.body;
 
-    this.pdfMetadata = new PDFMetadata(window_.PDFViewerApplication);
+    this.pdfMetadata = new PDFMetadata(pdfViewerApp);
 
     this.observer = new MutationObserver(debounce(() => this._update(), 100));
     this.observer.observe(this.pdfViewer.viewer, {
@@ -241,7 +246,7 @@ export class PDFIntegration {
     const pageCount = this.pdfViewer.pagesCount;
     for (let pageIndex = 0; pageIndex < pageCount; pageIndex++) {
       const page = this.pdfViewer.getPageView(pageIndex);
-      if (!page.textLayer?.renderingDone) {
+      if (!page?.textLayer?.renderingDone) {
         continue;
       }
 

src/types/annotator.js

@@ -138,7 +138,7 @@
  * when loaded in a frame that influence how it behaves.
  *
  * @typedef Globals
- * @prop {Object} [PDFViewerApplication] -
+ * @prop {import('./pdfjs').PDFViewerApplication} [PDFViewerApplication] -
  *   PDF.js entry point. If set, triggers loading of PDF rather than HTML integration.
  * @prop {boolean} [__hypothesis_frame] -
  *   Flag used to indicate that the "annotator" part of Hypothesis is loaded in

src/types/pdfjs.js

@@ -88,10 +88,14 @@
  *
  * Defined in `web/pdf_viewer.js` in the PDF.js source.
  *
+ * @prop {string} currentScaleValue - Zoom level/mode. This can be a string representation
+ *   of a float or a special constant ("auto", "page-fit", "page-width" and more)
  * @prop {number} pagesCount
  * @prop {EventBus} eventBus -
  *   Reference to the global event bus. Added in PDF.js v1.6.210.
  * @prop {(page: number) => PDFPageView|null} getPageView
+ * @prop {HTMLElement} viewer - DOM element containing the main content of the document
+ * @prop {() => void} update - Re-render the current view
  */
 
 /**
@@ -102,12 +106,21 @@
  * @prop {(event: string, listener: Function) => void} off
  */
 
+/**
+ * Object containing references to various DOM elements that make up the PDF.js viewer UI,
+ * as well as a few other global objects used by the viewer.
+ *
+ * @typedef AppConfig
+ * @prop {HTMLElement} appContainer
+ */
+
 /**
  * The `PDFViewerApplication` global which is the entry-point for accessing PDF.js.
  *
  * Defined in `web/app.js` in the PDF.js source.
  *
  * @typedef PDFViewerApplication
+ * @prop {AppConfig} [appConfig] - Viewer DOM elements. Since v1.5.188.
  * @prop {EventBus} [eventBus] -
  *   Global event bus. Since v1.6.210. This is not available until the PDF viewer
  *   has been initialized. See `initialized` and `initializedPromise` properties.