title: property QtObject marginHints: QtObject { ... }
body: <p>Looking at the sources of KDE plasmoids, some of widgets declare <code>marginHints</code> property like <a href="https://github.com/KDE/plasma-workspace/blob/19144ab096468493e541867fea4abab2cc13c312/applets/systemtray/package/contents/ui/main.qml#L319" rel="nofollow noreferrer">this</a>:</p>
  
  <pre><code>    property QtObject marginHints: QtObject {
          property int left: Math.round(units.smallSpacing / 2)
          property int top: Math.round(units.smallSpacing / 2)
          property int right: Math.round(units.smallSpacing / 2)
          property int bottom: Math.round(units.smallSpacing / 2)
      }
  </code></pre>
  
  <p>While not explicitly referenced from anywhere, removing them actually breaks the layout.</p>
  
  <p>Is it even documented elsewhere? How this works? Is this some kind of QML magic?</p>