WHM plugins now open in a new browser window by default when you access them via WHM's Manage Plugins interface (WHM Home >> cPanel >> Manage Plugins). Here are the corresponding entries in the cPanel 64 Change Log:
Implemented case CPANEL-11462: Open Munin in new window.
Fixed case CPANEL-12009: Updated code to open plugins in new tab.
This is planned as a temporary measure until some additional resources are offered to WHM plugin developers. The following feature request includes some information about progress towards an improved WHM theme: