[Impact] 
 * Upstream bug: https://bugzilla.xfce.org/show_bug.cgi?id=11893
   - Some applications, Eclipse in particular, may crash and be completely unusable because of a regression in xfwm4.
   - The issue is caused by xfwm4 4.12.1 monitoring the SubstructureNotifyMask event.
   - This is completely resolved in xfwm4 4.12.3. The offending event was not needed and is no longer monitored.

[Test Case]
 * Open Eclipse.
 * Go to Help > About.
 * Close the About dialog. Eclipse crashes.

[Regression Potential] 
 * Xfwm4 releases 4.12.2 and 4.12.3 are very minor and resolve a few bugs. Regression potential is minimal if not non-existent.