Left mouse button only for scrolling
Right mouse button for removing lines and opening properties of a behavior block.
middle mouse button for the circle pop-up (paste and import). When you move your cursor while holding the button down you will select an area.
We should also be able to drag and drop behavior blocks to the behavior screen. And Ctrl+Z to undo your last action (removing/adding lines or behavior blocks).
Then how would people with only a touchpad or a smartphone/tablet do anything but drag? Also, Apple Mac computers only have one or no click button. They have a weighted click, and right click is ctrl. 
My suggestion is for windows. Maybe left should be opening properties/removing lines and right should be scrolling (ctrl+mouse for mac). And middle click would be something like shift+click.
A button in the editor could give you 2 layout options, default and plus.