While still having the Trusted sites selected, press the
Custom level... button to change the security settings for the trusted sites.

Following settings are needed besides the Medium (default) settings:
Miscellaneous > Allow websites to open windows without address or status bar :
enableMiscellaneous > Use Phishing Filter :
disableMiscellaneous > Use Pop-up Blocker :
disable
Scripting > Allow programmatic clipboard acces :
enable
Press
OK to save your changes.