[mod] infinite_scroll as preference

* oscar theme: code from searx/plugins/infinite_scroll.py
* simple theme: new implementation

Co-authored-by: Markus Heiser <markus.heiser@darmarIT.de>

searx/static/themes/oscar/gruntfile.js

@@ -78,7 +78,7 @@ module.exports = function(grunt) {
       }
     },
     jshint: {
-      files: ['gruntfile.js', 'src/js/*.js', '../__common__/js/image_layout.js'],
+      files: ['gruntfile.js', 'src/js/*.js'],  // files in __common__ are linted by es lint in simple theme
       options: {
         reporterOutput: "",
         esversion: 6,

searx/static/themes/oscar/src/js/01_init.js

@@ -19,6 +19,7 @@ window.searxng = (function(d) {
 
     return {
         autocompleter: script.getAttribute('data-autocompleter') === 'true',
+        infinite_scroll: script.getAttribute('data-infinite-scroll') === 'true',
         method: script.getAttribute('data-method'),
         translations: JSON.parse(script.getAttribute('data-translations'))
     };

searx/static/themes/oscar/src/js/infinite_scroll.js

@@ -0,0 +1,50 @@
+/**
+ * @license
+ * (C) Copyright Contributors to the SearXNG project.
+ * (C) Copyright Contributors to the searx project (2014 - 2021).
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+$(document).ready(function() {
+    function hasScrollbar() {
+        var root = document.compatMode=='BackCompat'? document.body : document.documentElement;
+        return root.scrollHeight>root.clientHeight;
+    }
+
+    function loadNextPage() {
+        var formData = $('#pagination form:last').serialize();
+        if (formData) {
+            $('#pagination').html('<div class="loading-spinner"></div>');
+            $.ajax({
+                type: "POST",
+                url: $('#search_form').prop('action'),
+                data: formData,
+                dataType: 'html',
+                success: function(data) {
+                    var body = $(data);
+                    $('#pagination').remove();
+                    $('#main_results').append('<hr/>');
+                    $('#main_results').append(body.find('.result'));
+                    $('#main_results').append(body.find('#pagination'));
+                    if(!hasScrollbar()) {
+                        loadNextPage();
+                    }
+                }
+            });
+        }
+    }
+
+    if (searxng.infinite_scroll) {
+        var win = $(window);
+        $("html").addClass('infinite_scroll');
+        if(!hasScrollbar()) {
+            loadNextPage();
+        }
+        win.on('scroll', function() {
+            if ($(document).height() - win.height() - win.scrollTop() < 150) {
+                loadNextPage();
+            }
+        });
+    }
+
+});

searx/static/themes/oscar/src/less/infinite_scroll.less

@@ -0,0 +1,21 @@
+@keyframes rotate-forever {
+    0%   { transform: rotate(0deg) }
+    100% { transform: rotate(360deg) }
+}
+
+.loading-spinner {
+    animation-duration: 0.75s;
+    animation-iteration-count: infinite;
+    animation-name: rotate-forever;
+    animation-timing-function: linear;
+    height: 30px;
+    width: 30px;
+    border: 8px solid #666;
+    border-right-color: transparent;
+    border-radius: 50% !important;
+    margin: 0 auto;
+}
+
+html.infinite_scroll #pagination button {
+	visibility: hidden;
+}

searx/static/themes/oscar/src/less/logicodev-dark/oscar.less

@@ -4,6 +4,7 @@
 @import "../../../../__common__/less/result_templates.less";
 @import "../../less/result_templates.less";
 @import "../../less/preferences.less";
+@import "../infinite_scroll.less";
 @import "../../generated/pygments-logicodev.less";
 
 @stacked-bar-chart: rgb(213, 216, 215, 1);

searx/static/themes/oscar/src/less/logicodev/oscar.less

@@ -4,6 +4,7 @@
 @import "../../../../__common__/less/result_templates.less";
 @import "../../less/result_templates.less";
 @import "../../less/preferences.less";
+@import "../infinite_scroll.less";
 @import "../../generated/pygments-logicodev.less";
 
 @import "navbar.less";

searx/static/themes/oscar/src/less/pointhi/oscar.less

@@ -4,6 +4,7 @@
 @import "../../../../__common__/less/result_templates.less";
 @import "../../less/result_templates.less";
 @import "../../less/preferences.less";
+@import "../infinite_scroll.less";
 @import "../../generated/pygments-pointhi.less";
 
 @import "footer.less";