Currently it's impossible to add new expressions between two already existing
expressions by clicking with the mouse in the area between those two
expressions. It's not possible to add a new expression at the very top or at
the very bottom of the worksheet, neither. Mathemica and wxMaxima can do this.
In Cantor there is only the short-cut "ctrt+return" to add a new item after
the currently selected one. But this is not flexible enough. It would be great
to have a "tolerance area" of couple of pixels around each item. By hitting
this are with the mouse a new item should be produced.

It's much easier to click between
expressions to produce a new one then to go via the context menu. In text
processing you don't go via the context menu if you want to add new letters
between already existing letters - you just click in the text and start
typing at the place where you want to add new text. Similar for CAS, you click
between two expressions, you set your cursor so to say, and then a new field
for expression is produced when you start typing.

http://imgur.com/AokSEko
In this Mathematica screenshot you can see the horizontal line between two expressions.
Such a line indicates where your "expression cursor" currently is. If you
start typing, a new input will be produced at this place:
http://imgur.com/mNbsZdq

Similarly it is solved in wxMaxima, too.