Response from Carol Wong of Navicat Premium
Thanks for your review. You can set Navicat to open a new windows in Main Window. Please follow below step:
Windows: Go to Tools > Options > Docking > choose "to main window"
Mac: Go to Navicat's Preferences > Tabs > choose "Main window" for "Default open in".
About the interface issue, it will be improved in the next version :)