On the page that lists the disk usage of jobs or directories, the sidebar at the left looks like this:

The HTML structure of the sidebar starts like this (reindented):

<div id="side-panel">
  <div class="task ">
    <span class="task-link-wrapper ">
      <a href="/jenkins/" class="task-link ">
        <span class="task-icon-link">

In contrast, the nicer-looking sidebar of the Jenkins dashboard starts like this (reindented):

<div id="side-panel">
  <div id="tasks">
    <div class="task ">
      <span class="task-link-wrapper ">
        <a href="/jenkins/view/all/newJob" class="task-link ">
          <span class="task-icon-link">

If I edit the DOM of the disk usage sidebar in the Web browser and interpose <div id="tasks"> between <div id="side-panel"> and <div class="task ">, then it looks like this:

which at least aligns the icons and text better, but there is a strange-looking gray shading because of the "task-link--active" class. I'm not sure whether that is a side effect of the DOM tweaking.

Anyhow, I assume this corresponds to https://github.com/jenkinsci/cloudbees-disk-usage-simple-plugin/blob/170.va_fd5b_4ee6858/src/main/resources/com/cloudbees/simplediskusage/QuickDiskUsagePlugin/sidepanel.jelly using <l:task> without <l:tasks>, contrary to the documentation in https://github.com/jenkinsci/jenkins/blob/jenkins-2.346.3/core/src/main/resources/lib/layout/task.jelly.