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 is
loaded/restored before the viewer setup is 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

@@ -243,9 +243,10 @@ function matchingAncestorElement(el, context, selector) {
 
 const block_path = `${root}/catch/external`;
 
-function blockLink(target) {
-  const encodedHref = encodeURIComponent(target.href);
-  target.setAttribute("href", block_path + "?source=" + encodedHref);
+function blockLink(url) {
+  return viewerSettings.linkBlockingEnabled
+       ? block_path + "?source=" + encodeURIComponent(url)
+       : url;
 }
 
 function isExternalUrl(url) {
@@ -262,10 +263,8 @@ 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);
-      }
+      target.setAttribute("href", blockLink(target.href));
+      contentIframe.contentWindow.parent.postMessage({ externalURL : target.href }, "*");
     }
   }
 }
@@ -313,8 +312,8 @@ let viewerSetupComplete = false;
 function on_content_load() {
   if ( viewerSetupComplete ) {
     handle_content_url_change();
-    setup_external_link_blocker();
   }
+  setup_external_link_blocker();
 }
 
 function htmlDecode(input) {
@@ -446,6 +445,13 @@ function changeUILanguage() {
   });
 }
 
+function handleMessage(event) {
+  console.log("handleMessage");
+  if ( event.data.externalURL ) {
+    window.location = event.data.externalURL;
+  }
+}
+
 function setupViewer() {
   // Defer the call of handle_visual_viewport_change() until after the
   // presence or absence of the taskbar as determined by this function
@@ -453,6 +459,7 @@ function setupViewer() {
   setTimeout(handle_visual_viewport_change, 0);
 
   window.onresize = handle_visual_viewport_change;
+  window.addEventListener("message", handleMessage);
 
   const kiwixToolBarWrapper = document.getElementById('kiwixtoolbarwrapper');
   if ( ! viewerSettings.toolbarEnabled ) {

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=d407a38a" },
   { 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=d407a38a" 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">