Guaranteed activation of external link blocking

This is a quickfix for the problem observed with external link blocking
during certain history navigation actions (when the cached iframe content was
loaded/restored before the viewer setup was completed).

Since external link blocking doesn't depend on the translations (that
are asynchronously loaded during the viewer setup) it can be performed
unconditionally. However, the current dependence of `on_content_load()`
on viewer setup has to be addressed too.

static/skin/viewer.js

@@ -23,7 +23,7 @@ function userUrl2IframeUrl(url) {
 }
 
 function getBookFromUserUrl(url) {
-  if ( url == '' ) {
+  if ( url == '' || url.startsWith('catch/external?') ) {
     return null;
   }
 
@@ -131,6 +131,10 @@ function iframeUrl2UserUrl(url, query) {
     return '';
   }
 
+  if ( url == `${root}/catch/external` ) {
+    return `catch/external${query}`;
+  }
+
   if ( url == `${root}/search` ) {
     return `search${query}`;
   }
@@ -220,12 +224,16 @@ function handle_location_hash_change() {
 function handle_content_url_change() {
   const iframeLocation = contentIframe.contentWindow.location;
   console.log('handle_content_url_change: ' + iframeLocation.href);
-  document.title = contentIframe.contentDocument.title;
   const iframeContentUrl = iframeLocation.pathname;
   const iframeContentQuery = iframeLocation.search;
   const newHash = iframeUrl2UserUrl(iframeContentUrl, iframeContentQuery);
-  history.replaceState(viewerState, null, makeURL(location.search, newHash));
-  updateCurrentBookIfNeeded(newHash);
+  if ( newHash.startsWith('catch/external?') ) {
+    handleInterceptedExternalLink(newHash);
+  } else {
+    document.title = contentIframe.contentDocument.title;
+    history.replaceState(viewerState, null, makeURL(location.search, newHash));
+    updateCurrentBookIfNeeded(newHash);
+  }
 };
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -262,10 +270,7 @@ function onClickEvent(e) {
   const target = matchingAncestorElement(e.target, iframeDocument, "a");
   if (target !== null && "href" in target) {
     if ( isExternalUrl(target.href) ) {
-      target.setAttribute("target", "_top");
-      if ( viewerSettings.linkBlockingEnabled ) {
-        return blockLink(target);
-      }
+      return blockLink(target);
     }
   }
 }
@@ -304,6 +309,24 @@ function setup_external_link_blocker() {
   setupEventHandler(contentIframe.contentDocument, 'a', 'click', onClickEvent);
 }
 
+function getBlockedUrl(catchExternalUrl) {
+  const p = new URLSearchParams(catchExternalUrl.split('?')[1]);
+  return p.get('source');
+}
+
+function handleInterceptedExternalLink(catchExternalUrl) {
+  // The external link blocking page was loaded in the viewer iframe.
+  // We need to get rid of the viewer taskbar and load in the top frame either
+  // the external resource or, if running in --blockexternal mode, the
+  // confirmation page
+  const url = viewerSettings.linkBlockingEnabled
+            ? `${root}/` + catchExternalUrl
+            : getBlockedUrl(catchExternalUrl);
+  history.back(); // drop from the browsing history the state where the
+                  // external link catcher page is loaded in the iframe ...
+  window.location = url; // ... and load the target in the top frame instead
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // End of external link blocking
 ////////////////////////////////////////////////////////////////////////////////
@@ -313,8 +336,8 @@ let viewerSetupComplete = false;
 function on_content_load() {
   if ( viewerSetupComplete ) {
     handle_content_url_change();
-    setup_external_link_blocker();
   }
+  setup_external_link_blocker();
 }
 
 function htmlDecode(input) {

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=db02f59d" },
   { 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=b00b12db" defer></script>
-    <script type="text/javascript" src="./skin/viewer.js?cacheid=b9a574d4" defer></script>
+    <script type="text/javascript" src="./skin/viewer.js?cacheid=db02f59d" 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">