JQuery-ui accordion in fill mode and overflow - extraneous 2px

Question

In this fiddle PoC, I need to decrease the height of my JQuery-ui accordion by two additional unexplained pixels if I want to get rid of the overflow vertical scrollbar.

• I'm in 'fill' heightStyle mode as seen below.
• The height of my accordion is the height of the body minus the height of the footer and the header. Minus 2 pixels.
• all libs are in latest versions (at the time of writing) except jquery not in '3'.
• I know about accordion.refresh() and css flex but I'm curious about the "why", and I'd rather use calc() rather before I stuff my CSS with flex boxes.

$( function() {
    $( "#accordion" ).accordion({heightStyle: 'fill' });
  } );

html, body {
    height: 100%;
  }
  :root{
    --header-height: 40px;
    --footer-height: 40px;
    --two-unexplained-px: 2px;
  }
  body {
    overflow:auto;
    background-color: red;
  }
  * {
    margin: 0px;
    padding: 0px;
  }
  #header, #footer {
    background-color: #adf;
  }
  #header {
    height: var(--header-height);
  }
  #footer {
    height: var(--footer-height);
  }
  #accordion {
    height: calc(100% - var(--header-height) - var(--footer-height) - var(--two-unexplained-px));
  }

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/2.2.4/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jqueryui/1.12.1/jquery-ui.min.js"></script>
<link href="https://cdnjs.cloudflare.com/ajax/libs/jqueryui/1.12.1/jquery-ui.min.css" rel="stylesheet"/>
<div id="header">Header here</div>
 <div id="accordion"><h2><a>Work here</a></h2><div>Text Here</div></div>
 <div id="footer">Footer here</div>

Answer 1

The two additional pixels come from this rule:

.ui-accordion .ui-accordion-header {
    margin: 2px 0 0 0;
}