Recently my SW 2014 display pane is blank, just showing background, unless I specifically click on a feature (i.e. Boss-Extrude) in the left pane, or click on a sub-assembly in an assembly.  I went to Options-->Colors and selected "Reset Colors to Defaults" but didn't help.  This happens on both parts and assemblies.  What might have happened? How can I get this back to previous working condition?