Add this as a global javascript widget in Zendesk to achieve collapsible category headers.

collapse_categories.js

var collapsed_by_default = false;  // set this to true if you prefer collapsed by default

function HideCategory(id, IsAgent, animation) {
  // Function hides category
  $j("div#category_" + id).hide(animation);
  SetDownArrow(id, IsAgent);
  
  $j.cookie('hide_category_' + id, 'yes', { expires: 180 });
  $j("div#category_header_" + id).attr("onclick", "ShowCategory(" + id + "," +IsAgent+ ",'blind')");
};

function ShowCategory(id, IsAgent, animation) {
  // Function shows category
  $j("div#category_" + id).show(animation);
  SetUpArrow(id, IsAgent);

  $j.cookie('hide_category_' + id, 'no', { expires: 180 });
  $j("div#category_header_" + id).attr("onclick", "HideCategory(" + id + "," +IsAgent+ ",'blind')");
};

function SetUpArrow(id, IsAgent){
  // Show Up arrow.
  // Script uses image from Zendesk resources https://www.zendesk.com/wp-content/themes/zendesk-twentyeleven/img/support/icons-toggle.png
  // Better to store it in other place
  
  // Trick: agent's markup is different than end-users. We should insert arrow by different ways
  if (IsAgent==1)
  {
    $j("div#category_header_" + id).children('div.category-name').css('background','#f8f8f8 url(https://www.zendesk.com/wp-content/themes/zendesk-twentyeleven/img/support/icons-toggle.png) no-repeat right 5px');
  } else {
    $j("div#category_header_" + id).children('div.category-header').css('background','#f8f8f8 url(https://www.zendesk.com/wp-content/themes/zendesk-twentyeleven/img/support/icons-toggle.png) no-repeat 90% 18px');
  }
}

function SetDownArrow(id, IsAgent){
  // Show Down arrow.
  // Script uses image from Zendesk resources https://www.zendesk.com/wp-content/themes/zendesk-twentyeleven/img/support/icons-toggle.png
  // Better to store it in other place
  
  // Trick: agent's markup is different than end-users. We should insert arrow by different ways
  if (IsAgent==1)
  {
    $j("div#category_header_" + id).children('div.category-name').css('background','#f8f8f8 url(https://www.zendesk.com/wp-content/themes/zendesk-twentyeleven/img/support/icons-toggle.png) no-repeat right -50px');
  } else {
    $j("div#category_header_" + id).children('div.category-header').css('background','#f8f8f8 url(https://www.zendesk.com/wp-content/themes/zendesk-twentyeleven/img/support/icons-toggle.png) no-repeat 90% -37px');
  }
}

// Get current url
var s_url = window.location.pathname;
var s_urlparts = s_url.split('/');
var s_section = s_urlparts[1];

// Run if we in the "forums" section
if (s_section == 'forums' || s_section == 'home') {

  $j("span.follow_link").hide(); //remove follow link
  // Get all sections from page
  var cat_headers = $j("div.category-header");
  $j.each(cat_headers, function () {

    var base = $j(this).children("div.category-name");

    var IsAgent = 1;
    if (base.length == 0) {
      // Is non agent if that
      base = $j(this);
      IsAgent=0;
    }

    var child = base.children("h2").children("a");

    if (child.length != 0) {
      // "0" means No Category Section
      var id = child.attr("href")
      var id_splitted = id.split("/");
      if (id_splitted.length == 3) {
        var id_value = id_splitted[2].split("-")[0];

        base.parent().attr("onclick","HideCategory(" + id_value + "," +IsAgent+ ")");
        if (IsAgent==0) {
          // Trick: we should add ID because it absent in end-user markup
          base.parent().attr("id","category_header_" + id_value);
        }

        // underline links
        child.mouseover(function(){$j(this).css("text-decoration","underline")});
        child.mouseleave(function(){$j(this).css("text-decoration","none")});
  
        base.parent().css("cursor","pointer");
  
        var ckie = $j.cookie('hide_category_' + id_value);
        if ((ckie == 'yes') || (collapsed_by_default && !(ckie))) {
          HideCategory(id_value, IsAgent);
        } else {
          ShowCategory(id_value, IsAgent);
        }
      }
    }
  });
}