For several releases of PB6b the history bar left arrow hangs after the tool windows are deactivated and full plex is restored. The next action tends to correct it.
Java 32bit Version 8 Update 141
Firefox, Office 2013 Pro Plus 32bit
64bit Primary Laptop, 8GB RAM, Intel Core i7
64bit Secondary Laptop, 64GB RAM, Intel Xeon E3
Brain user since 1997