If you have more than one monitors and on one of them you maximize a ToolWindow, than dock another pane and undock it, the second pane's width will be equal to the width of the two monitors.
Available in LIB version 2015.2.713, it will be also available in the 2015 Q2 SP1.
The issue is still present (Q2 2015) in a different scenario - when the second ToolWindow is docked in of the side compasses of the first one (the maximized one).
Available in LIB version 2014.3.1409 as it will be also available in the 2015 Q1.