What sets of boxes can be selected?

Question

In this question I am only interested in mathematical expressions arising in output cells, not more elaborate code which is typically found in input cells. My understanding is that such output is made of boxes which, like every expression in Mathematica, have a the structure of a tree.

I would have naively thought that what can be selected in an output cell is any subtree of that tree. E.g. in the fraction x+y over z (with box form FractionBox[RowBox[{"x","+","y"}],"z"]), one cannot select y and z without selecting the whole fraction.

However, this is too naive: MakeBoxes[-x y] gives the nested structure RowBox[{RowBox[{-, x}], , y}], but one can nevertheless select x and y in the output of -x y without selecting -.

Another similar case is that MakeBoxes[ab cd] gives RowBox[{"ab"," ","cd"}] but one can select (in the output of ab cd) b c, whose box representation is not a subtree.

I think my question boils down to: is RowBox is the only case where one can select a non-subtree?

Answer 1

You can get this behaviour by setting StucturedSelection to True at the Global`, Notebook, or Cell level. See the Options for Expression Input and Output guide.

This can be done with the Options Inspector or with SetOptions. To set it at the notebook level:

SetOptions[EvaluationNotebook[], StructuredSelection -> True]

After this is done you will only be able to select complete sub-expressions in the notebook.

Hope this helps.