Win10, Opera Browser 70.0.3701.0:
When the screen (1366px) is split to half width (winkey+<left>), the browser has half the width which causes the mentioned bar to keep oscillating back and forth between the two positions shown in the image.
The below div appears to ontain the issue.
<div id="p-views" role="navigation" class="vectorTabs" aria-labelledby="p-views-label">
<h3 id="p-views-label">Views</h3>
<ul>
<li id="ca-view" class="collapsible selected"><a href="/wiki/Wigner_semicircle_distribution">Read</a></li><li id="ca-edit" class="collapsible"><a href="/w/index.php?title=Wigner_semicircle_distribution&action=edit" title="Edit this page [alt-shift-e]" accesskey="e">Edit</a></li><li id="ca-history" class="collapsible" style="display: block;"><a href="/w/index.php?title=Wigner_semicircle_distribution&action=history" title="Past revisions of this page [alt-shift-h]" accesskey="h">View history</a></li>
</ul>
</div>