Only load PDF.js style overrides in PDF.js

Only load the `pdfjs-overrides.css` bundle when the client is loaded in
PDF.js.

This fixes an issue where a global styling change that was recently
added specifically for PDF.js [1] ended up affecting other web pages
[2].

[1] https://github.com/hypothesis/client/pull/4208
[2] https://github.com/hypothesis/product-backlog/issues/1270#issuecomment-1039647361

src/boot/boot.js

@@ -28,6 +28,10 @@ const commonPolyfills = [
  *   A mapping from canonical asset path to cache-busted asset path
  */
 
+/**
+ * @typedef {Window & { PDFViewerApplication?: object }} MaybePDFWindow
+ */
+
 /**
  * Mark an element as having been added by the boot script.
  *
@@ -197,17 +201,23 @@ export function bootHypothesisClient(doc, config) {
 
   const polyfills = polyfillBundles(commonPolyfills);
 
+  // Override styling in certain document viewers.
+  const viewerStyles = [];
+  if (
+    /** @type {MaybePDFWindow} */ (window).PDFViewerApplication !== undefined
+  ) {
+    viewerStyles.push('styles/pdfjs-overrides.css');
+  }
+
   injectAssets(
     doc,
     config,
     [
-      // Vendor code and polyfills
       ...polyfills,
+      ...viewerStyles,
 
       'scripts/annotator.bundle.js',
-
       'styles/highlights.css',
-      'styles/pdfjs-overrides.css',
     ],
 
     // Force re-evaluation of JS module scripts, so that the annotator entry

src/boot/test/boot-test.js

@@ -104,12 +104,29 @@ describe('bootstrap', () => {
       const expectedAssets = [
         'scripts/annotator.bundle.1234.js#ts=123',
         'styles/highlights.1234.css',
-        'styles/pdfjs-overrides.1234.css',
       ].map(assetUrl);
 
       assert.deepEqual(findAssets(iframe.contentDocument), expectedAssets);
     });
 
+    it('loads styling overrides in PDF.js', () => {
+      clock.tick(123); // Set timestamp used by module cache-busting fragment.
+
+      window.PDFViewerApplication = {};
+      try {
+        runBoot('annotator');
+        const expectedAssets = [
+          'scripts/annotator.bundle.1234.js#ts=123',
+          'styles/highlights.1234.css',
+          'styles/pdfjs-overrides.1234.css',
+        ].map(assetUrl);
+
+        assert.deepEqual(findAssets(iframe.contentDocument), expectedAssets);
+      } finally {
+        delete window.PDFViewerApplication;
+      }
+    });
+
     it('preloads assets used wihin shadow roots in the annotation layer', () => {
       runBoot('annotator');