welcome to the Polycom Community.
The Idle browser should work independently from the Screensaver especially as this option does not make much sense on a phone like the VVX300.
I just did a quick test in my lab with a development signed VVX410 and can confirm that without activating the screensaver the idle browser does not appear.
Please raise this via your Polycom reseller and / or Polycom support directly and reference this post and my name to follow this up internally.
Please ensure to provide some feedback if this reply has helped you so other users can profit from your experience.
Polycom Global Services
this is working as designed.
Due the non touch screen interface (unable to swipe) the IDLE Browser must be used in conjunction with the Screensaver on the VVX3x0 / VVX4x0 range.