PDF-friendly book home button in the viewer

In firefox, when PDF content is displayed in the viewer, changing the
viewer URL in the address bar had no effect. The most prominent
manifestation of this bug was the broken book home button but the same
issue was present even if the fragment component of the viewer URL was
edited manually. The bug was a result of

1. an optimization preventing any actions if the new content URL is the
   same as the old content URL (this was needed to break the infinite loop
   of mutual updates of the top-window and content window/iframe URLs when
   any one of them changes).

2. sandboxing of the iframe and inability to access the content URL in
   iframe because of cross-origin restrictions when the content is a PDF
   displayed by the builtin viewer.

Now that issue is fixed. A slight remaining defect is that the
addressbar URL is still not updated when a PDF file is loaded/displayed
in the viewer.

static/skin/viewer.js

@@ -204,14 +204,29 @@ function handle_visual_viewport_change() {
   contentIframe.height = wh - contentIframe.offsetTop - 4;
 }
 
+function setIframeUrl(path) {
+  try {
+    // Don't do anything if we are already at the requested URL.
+    // This is needed to break the infinite ping-pong played by
+    // handle_location_hash_change() and handle_content_url_change()
+    // (when either top-window or content window/iframe URL changes the other
+    // one is updated too).
+    if ( path == contentIframe.contentWindow.location.pathname )
+      return;
+  } catch ( error ) {
+    // This happens in Firefox when a PDF file is displayed in the iframe
+    // (caused by sandboxing of the iframe and cross-origin mismatch with
+    // the builtin PDF viewer).
+    // Fall through and load the requested URL.
+  }
+  contentIframe.contentWindow.location.replace(path);
+}
+
 function handle_location_hash_change() {
   const hash = window.location.hash.slice(1);
   console.log("handle_location_hash_change: " + hash);
   updateCurrentBookIfNeeded(hash);
-  const iframeContentUrl = userUrl2IframeUrl(hash);
-  if ( iframeContentUrl != contentIframe.contentWindow.location.pathname ) {
-    contentIframe.contentWindow.location.replace(iframeContentUrl);
-  }
+  setIframeUrl(userUrl2IframeUrl(hash));
   updateSearchBoxForLocationChange();
   previousScrollTop = Infinity;
   history.replaceState(viewerState, null);

test/server.cpp

@@ -73,7 +73,7 @@ const ResourceCollection resources200Compressible{
   { DYNAMIC_CONTENT, "/ROOT%23%3F/skin/taskbar.css" },
   { STATIC_CONTENT,  "/ROOT%23%3F/skin/taskbar.css?cacheid=bbdaf425" },
   { DYNAMIC_CONTENT, "/ROOT%23%3F/skin/viewer.js" },
-  { STATIC_CONTENT,  "/ROOT%23%3F/skin/viewer.js?cacheid=b9a574d4" },
+  { STATIC_CONTENT,  "/ROOT%23%3F/skin/viewer.js?cacheid=3330eca2" },
   { DYNAMIC_CONTENT, "/ROOT%23%3F/skin/fonts/Poppins.ttf" },
   { STATIC_CONTENT,  "/ROOT%23%3F/skin/fonts/Poppins.ttf?cacheid=af705837" },
   { DYNAMIC_CONTENT, "/ROOT%23%3F/skin/fonts/Roboto.ttf" },
@@ -312,7 +312,7 @@ R"EXPECTEDRESULT(    <link type="text/css" href="./skin/taskbar.css?cacheid=bbda
     <link type="text/css" href="./skin/css/autoComplete.css?cacheid=08951e06" rel="Stylesheet" />
     <script type="module" src="./skin/i18n.js?cacheid=2cf0f8c5" defer></script>
     <script type="text/javascript" src="./skin/languages.js?cacheid=648526e1" defer></script>
-    <script type="text/javascript" src="./skin/viewer.js?cacheid=b9a574d4" defer></script>
+    <script type="text/javascript" src="./skin/viewer.js?cacheid=3330eca2" defer></script>
     <script type="text/javascript" src="./skin/autoComplete.min.js?cacheid=1191aaaf"></script>
       const blankPageUrl = root + "/skin/blank.html?cacheid=6b1fa032";
             <img src="./skin/langSelector.svg?cacheid=00b59961">