Avoid flicker when highlighting a section

Previously, elements were wrapped in a div on hover and unwrapped when
the mouse was moved again. This generally worked okay, but led to
problems where the wrapped section contained elements with their own
rendering context (video, object, iframe, ...). The wrapping caused
rerendering of these elements resulting in flicker and layoutshift:

splitbrain/dokuwiki-plugin-vshare#13
cosmocode/dokuwiki-plugin-diagrams#4

This patch changes the higlighting mechanism to always wrap *all*
sections in a div and only toggle classes on hover. Class names have
been chosen to be backwardscompatible, so this should continue to work
as before in all templates.

lib/scripts/page.js

@@ -21,29 +21,37 @@ dw_page = {
      */
     sectionHighlight: function() {
         jQuery('form.btn_secedit')
-            .on('mouseover', function(){
-                var $tgt = jQuery(this).parent(),
-                    nr = $tgt.attr('class').match(/(\s+|^)editbutton_(\d+)(\s+|$)/)[2],
-                    $highlight = jQuery(),                                             // holder for elements in the section to be highlighted
-                    $highlightWrap = jQuery('<div class="section_highlight"></div>');  // section highlight wrapper
+            /*
+             * wrap the editable section in a div
+             */
+            .each(function () {
+                let $tgt = jQuery(this).parent();
+                const nr = $tgt.attr('class').match(/(\s+|^)editbutton_(\d+)(\s+|$)/)[2];
+                let $highlight = jQuery(); // holder for elements in the section to be highlighted
+                const $highlightWrap = jQuery('<div class="section_highlight_wrapper"></div>');
+
+                // the edit button should be part of the highlight
+                $highlight = $highlight.add($tgt);
 
                 // Walk the dom tree in reverse to find the sibling which is or contains the section edit marker
-                while($tgt.length > 0 && !($tgt.hasClass('sectionedit' + nr) || $tgt.find('.sectionedit' + nr).length)) {
+                while ($tgt.length > 0 && !($tgt.hasClass('sectionedit' + nr) || $tgt.find('.sectionedit' + nr).length)) {
                     $tgt = $tgt.prev();
                     $highlight = $highlight.add($tgt);
                 }
-              // insert the section highlight wrapper before the last element added to $highlight
-              $highlight.filter(':last').before($highlightWrap);
-              // and move the elements to be highlighted inside the section highlight wrapper
-              $highlight.detach().appendTo($highlightWrap);
+                // wrap the elements to be highlighted in the section highlight wrapper
+                $highlight.wrapAll($highlightWrap);
             })
-            .on('mouseout', function(){
-                // find the section highlight wrapper...
-                var $highlightWrap = jQuery('.section_highlight');
-                // ...move its children in front of it (as siblings)...
-                $highlightWrap.before($highlightWrap.children().detach());
-                // ...and remove the section highlight wrapper
-                $highlightWrap.detach();
+            /*
+             * highlight the section
+             */
+            .on('mouseover', function () {
+                jQuery(this).parents('.section_highlight_wrapper').addClass('section_highlight');
+            })
+            /*
+             * remove highlight
+             */
+            .on('mouseout', function () {
+                jQuery(this).parents('.section_highlight_wrapper').removeClass('section_highlight');
             });
     },