title: Datatables header misalignment
body: <p>The picture below shows the header of the datatable misalignment before you click on the textbox (search/filter..). once the latter becomes in focus, the header aligns again. I noticed that when scrollY is off the header is fine, i need it on. any idea how to fix it.
  in the following snippets, just change the paging option and you can see the realignment of the header again.</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>$('#RegSrc').DataTable({
     dom:"&lt;'row'&lt;'#tblnoitfy'&gt;&gt;&lt;'row'&lt;'col-sm-4'l&gt;&lt;'col-sm-8'p&gt;&gt;" + "&lt;'row'&lt;'col-sm-12'tr&gt;&gt;",
              select: true,
              scrollY: 200,
              deferRender: true,
              //scrollY:     500,
              responsive: false,
              fixedHeader: {
                  header: true,
                  footer: true
              },
              "processing": true,
              "serverSide": false,
              bAutoWidth: true,
              data: [],
              rowCallback: function (row, data) { },
    });
  
  $("#btn").click(function() {
    $("#mdl").dockmodal();
  })</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;link href="https://cdn.datatables.net/1.10.15/css/dataTables.bootstrap.min.css" rel="stylesheet"/&gt;
  &lt;link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet"/&gt;
  &lt;script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"&gt;&lt;/script&gt;
  &lt;script src="https://cdn.datatables.net/1.10.15/js/jquery.dataTables.min.js"&gt;&lt;/script&gt;
  &lt;script src="http://uxmine.com/demo/dockmodal/assets/js/jquery.dockmodal.js"&gt;&lt;/script&gt;
  &lt;script src="https://cdn.datatables.net/1.10.15/js/dataTables.bootstrap.min.js"&gt;&lt;/script&gt;
  &lt;link href="http://uxmine.com/demo/dockmodal/assets/css/jquery.dockmodal.css" rel="stylesheet"/&gt;
  &lt;input id="btn" type="button" value="Click ME!" /&gt;
  
  
  &lt;div style="display:none"&gt;
    &lt;div id="mdl" class="panel-body"&gt;
      &lt;input id="RegSrcsrcctl" type="text" /&gt;
      &lt;input id="bt1" type="button" value="dummy search" /&gt;
      &lt;table id="RegSrc" class="table table-bordered table-striped table-condensed mb-none" style="width:100%"&gt;
        &lt;thead&gt;
          &lt;tr&gt;
            &lt;th&gt;&lt;/th&gt;
            &lt;th&gt;&lt;b&gt;File Number&lt;/b&gt;&lt;/th&gt;
            &lt;th&gt;&lt;b&gt;Patient Name&lt;/b&gt;&lt;/th&gt;
            &lt;th&gt;&lt;b&gt;DOB&lt;/b&gt;&lt;/th&gt;
            &lt;th&gt;&lt;b&gt;Age&lt;/b&gt;&lt;/th&gt;
          &lt;/tr&gt;
        &lt;/thead&gt;
        &lt;tbody&gt;
        &lt;/tbody&gt;
      &lt;/table&gt;
    &lt;/div&gt;
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p><a href="https://i.stack.imgur.com/W3y1e.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/W3y1e.jpg" alt="enter image description here"></a></p>