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Try turning off compositing (Settings Manager -> Window Manager
Tweaks -> Compositor). Does that get rid of the square? Does it
come back if you re-enable Compositor?<br>
<br>
This worked for me on a similar problem in 12.04--thanks to <a
id="yui-gen52" class="username online popupctrl"
href="http://ubuntuforums.org/member.php?u=27546" title="Toz is
online now"><strong><span style="font-weight: bold; color:
#DD4814;">Toz</span></strong></a> at Ubuntu Forums for my
solution.<br>
<br>
<div class="moz-cite-prefix">On 09/30/2014 05:00 AM,
<a class="moz-txt-link-abbreviated" href="mailto:xubuntu-users-request@lists.ubuntu.com">xubuntu-users-request@lists.ubuntu.com</a> wrote:<br>
</div>
<blockquote
cite="mid:mailman.163.1412078412.7554.xubuntu-users@lists.ubuntu.com"
type="cite">
<pre wrap="">I still have the black rectangle on my desktop.
</pre>
</blockquote>
<br>
<div class="moz-signature">-- <br>
<font face="Ubuntu">
<p style="color:gray"><i>Not</i> "sent from my iphone".</p>
</font></div>
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