I have noticed that windows from certain applications behave strangely or become unresponsive in XFCE 4.12.
The best example I have is the Cisco WebEx client. The window that displays the Shared Desktop routinely becomes unresponsive and needs to be killed. In particular, it seems that if another window is given focus for any meaningful length of time, the Shared Desktop window ceases to update and is unresponsive if you then try to close it gracefully.
This behavior is extremely repeatable.
In KDE 4.14 the same application works without issue and I am pretty sure (although not 100% certain) that I used it in the past in XFCE 4.10 without problems.
I have also noticed the Google Hangout video plugin seems not to redraw smoothly. Again, it has no problems in KDE.
I have tried disabling compositing in XFCE, but that did not help.
This is on a laptop running Slackware 14.1 (updated with XFCE 4.12) with Intel 4400 graphics chipset.
If the problem occurs even without compositing enabled, then it's unlikely related to the window manager, each application being responsible to refresh its content.
Anyhow I would need a simple (ie not involving a closed source application) and reliable way to reproduce to be able to investigate, as it is, this is a tad too vague to investigate.