Open external links in new tabs, and warn the user

www/index.html

@@ -476,6 +476,13 @@ <h3>Display settings</h3>
                                         <strong>Use Home key to focus search bar</strong> (may rarely have side effects on ZIM files that handle Home key)
                                     </label>
                                 </div>
+                                <div class="checkbox">
+                                    <label title="Opens the external links outside kiwix-js (avoids some side-effects affecting kiwix-js UI).">
+                                        <input type="checkbox" name="openExternalLinksInNewTabs"
+                                               id="openExternalLinksInNewTabsCheck" checked>
+                                        <strong>Open external links in new tabs</strong>. Disabling this might break kiwix-js UI in some specific cases
+                                    </label>
+                                </div>
                                 <div class="form-group">
                                     <label title="Allows selection of themes either for the app only, or for the app and the loaded content.">
                                         <b>Select app theme</b> (content inversion is experimental):

www/js/app.js

@@ -90,6 +90,8 @@ define(['jquery', 'zimArchiveLoader', 'uiUtil', 'settingsStore','abstractFilesys
     params['appTheme'] = settingsStore.getItem('appTheme') || 'light'; // Currently implemented: light|dark|dark_invert|dark_mwInvert|auto|auto_invert|auto_mwInvert|
     // A global parameter to turn on/off the use of Keyboard HOME Key to focus search bar
     params['useHomeKeyToFocusSearchBar'] = settingsStore.getItem('useHomeKeyToFocusSearchBar') === 'true';
+    // A global parameter to turn on/off opening external links in new tab (for ServiceWorker mode)
+    params['openExternalLinksInNewTabs'] = settingsStore.getItem('openExternalLinksInNewTabs') ? settingsStore.getItem('openExternalLinksInNewTabs') === 'true' : true;
     // A parameter to access the URL of any extension that this app was launched from
     params['referrerExtensionURL'] = settingsStore.getItem('referrerExtensionURL');
     // A parameter to set the content injection mode ('jquery' or 'serviceworker') used by this app
@@ -162,6 +164,7 @@ define(['jquery', 'zimArchiveLoader', 'uiUtil', 'settingsStore','abstractFilesys
     document.getElementById('titleSearchRangeVal').textContent = params.maxSearchResultsSize;
     document.getElementById('appThemeSelect').value = params.appTheme;
     document.getElementById('useHomeKeyToFocusSearchBarCheck').checked = params.useHomeKeyToFocusSearchBar;
+    document.getElementById('openExternalLinksInNewTabsCheck').checked = params.openExternalLinksInNewTabs;
     switchHomeKeyToFocusSearchBar();
     document.getElementById('bypassAppCacheCheck').checked = !params.appCache;
     document.getElementById('appVersion').innerHTML = 'Kiwix ' + params.appVersion;
@@ -478,6 +481,10 @@ define(['jquery', 'zimArchiveLoader', 'uiUtil', 'settingsStore','abstractFilesys
         settingsStore.setItem('useHomeKeyToFocusSearchBar', params.useHomeKeyToFocusSearchBar, Infinity);
         switchHomeKeyToFocusSearchBar();
     });
+    $('input:checkbox[name=openExternalLinksInNewTabs]').on('change', function () {
+        params.openExternalLinksInNewTabs = this.checked ? true : false;
+        settingsStore.setItem('openExternalLinksInNewTabs', params.openExternalLinksInNewTabs, Infinity);
+    });
     document.getElementById('appThemeSelect').addEventListener('change', function (e) {
         params.appTheme = e.target.value;
         settingsStore.setItem('appTheme', params.appTheme, Infinity);
@@ -1456,6 +1463,22 @@ define(['jquery', 'zimArchiveLoader', 'uiUtil', 'settingsStore','abstractFilesys
                     // Configure home key press to focus #prefix only if the feature is in active state
                     if (params.useHomeKeyToFocusSearchBar)
                         iframeArticleContent.contentWindow.addEventListener('keydown', focusPrefixOnHomeKey);
+                    if (params.openExternalLinksInNewTabs) {
+                        // Add event listener to iframe window to check for links to external resources
+                        iframeArticleContent.contentWindow.addEventListener('click', function (event) {
+                            // Find the closest enclosing A tag (if any)
+                            var clickedAnchor = uiUtil.closestAnchorEnclosingElement(event.target);
+                            if (clickedAnchor) {
+                                var href = clickedAnchor.getAttribute('href');
+                                // We assume that, if an absolute http(s) link is hardcoded inside an HTML string,
+                                // it means it's a link to an external website.
+                                // We also do it for ftp even if it's not supported any more by recent browsers...
+                                if (/^(?:http|ftp)/i.test(href)) {
+                                    uiUtil.warnAndOpenExternalLinkInNewTab(event, clickedAnchor);
+                                }
+                            }
+                        });
+                    }
                     // Reset UI when the article is unloaded
                     iframeArticleContent.contentWindow.onunload = function () {
                         // remove eventListener to avoid memory leaks
@@ -1561,7 +1584,7 @@ define(['jquery', 'zimArchiveLoader', 'uiUtil', 'settingsStore','abstractFilesys
 
     // A string to hold any anchor parameter in clicked ZIM URLs (as we must strip these to find the article in the ZIM)
     var anchorParameter;
-
+    
     /**
      * Display the the given HTML article in the web page,
      * and convert links to javascript calls
@@ -1699,10 +1722,14 @@ define(['jquery', 'zimArchiveLoader', 'uiUtil', 'settingsStore','abstractFilesys
                 } else if (anchorTarget) {
                     // It's a local anchor link : remove escapedUrl if any (see above)
                     anchor.setAttribute('href', '#' + anchorTarget[1]);
-                } else if (anchor.protocol !== currentProtocol ||
-                    anchor.host !== currentHost) {
+                } else if ((anchor.protocol !== currentProtocol ||
+                    anchor.host !== currentHost) && params.openExternalLinksInNewTabs) {
                     // It's an external URL : we should open it in a new tab
-                    anchor.target = '_blank';
+                    anchor.addEventListener('click', function(event) {
+                        // Find the closest enclosing A tag
+                        var clickedAnchor = uiUtil.closestAnchorEnclosingElement(event.target);
+                        uiUtil.warnAndOpenExternalLinkInNewTab(event, clickedAnchor);
+                    });
                 } else {
                     // It's a link to an article or file in the ZIM
                     var uriComponent = uiUtil.removeUrlParameters(href);

www/js/lib/uiUtil.js

@@ -590,6 +590,59 @@ define(rqDef, function(settingsStore) {
 
     // If global variable webpMachine is true (set in init.js), then we need to initialize the WebP Polyfill
     if (webpMachine) webpMachine = new webpHero.WebpMachine({useCanvasElements: true});
+
+    /**
+     * Warn the user that he/she clicked on an external link, and open it in a new tab
+     * 
+     * @param {Event} event the click event (on an anchor) to handle
+     * @param {Element} clickedAnchor the DOM anchor that has been clicked (optional, defaults to event.target)
+     */
+    function warnAndOpenExternalLinkInNewTab(event, clickedAnchor) {
+        event.preventDefault();
+        event.stopPropagation();
+        if (!clickedAnchor) clickedAnchor = event.target;
+        var target = clickedAnchor.target;
+        var message = '<p>Do you want to open this external link?';
+        if (!target || target === '_blank') {
+            message += ' (in a new tab)';
+        }
+        message += '</p><p style="word-break:break-all;">' + clickedAnchor.href + '</p>';
+        systemAlert(message, 'Opening external link', true).then(function (response) {
+            if (response) {
+                if (!target)
+                    target = '_blank';
+                window.open(clickedAnchor.href, target);
+            }
+        });
+    }
+
+    /**
+     * Finds the closest <a> or <area> enclosing tag of an element.
+     * Returns undefined if there isn't any.
+     * 
+     * @param {Element} element
+     * @returns {Element} closest enclosing anchor tag (if any)
+     */
+    function closestAnchorEnclosingElement(element) {
+        if (Element.prototype.closest) {
+            // Recent browsers support that natively. See https://developer.mozilla.org/en-US/docs/Web/API/Element/closest
+            return element.closest('a,area');
+        } else {
+            // For other browsers, notably IE, we do that by hand (we did not manage to make polyfills work on IE11)
+            var currentElement = element;
+            while (currentElement.tagName !== 'A' && currentElement.tagName !== 'AREA') {
+                // If there is no parent Element, we did not find any enclosing A tag
+                if (!currentElement.parentElement) {
+                    return;
+                } else {
+                    // Else we try the next parent Element
+                    currentElement = currentElement.parentElement;
+                }
+            }
+            // If we reach this line, it means the currentElement is the enclosing Anchor we're looking for
+            return currentElement;
+        }
+    }
 
     /**
      * Functions and classes exposed by this module
@@ -610,6 +663,8 @@ define(rqDef, function(settingsStore) {
         removeAnimationClasses: removeAnimationClasses,
         applyAnimationToSection: applyAnimationToSection,
         applyAppTheme: applyAppTheme,
-        reportAssemblerErrorToAPIStatusPanel: reportAssemblerErrorToAPIStatusPanel
+        reportAssemblerErrorToAPIStatusPanel: reportAssemblerErrorToAPIStatusPanel,
+        warnAndOpenExternalLinkInNewTab: warnAndOpenExternalLinkInNewTab,
+        closestAnchorEnclosingElement: closestAnchorEnclosingElement
     };
 });