Please describe the problem:
If a row is appended to a TreeView and the new row isn't immediately visible and set_cursor is called on a cell in that row with editable = true, then the row will be selected, but editing will not start until set_cursor is called again.

Steps to reproduce:
1. Compile and run the attached program
g++ editnew.cc -o editnew `pkg-config gtkmm-2.4 --cflags --libs` && ./editnew
2. Add a new row and start editing it by pressing the "Edit" button.
This works at expected since the row is created inside the visible area
3. Populate the list
4. Repeat step 2.


Actual results:
The row is selected.

Expected results:
The cell should also enter "editing mode".

Does this happen every time?
Yes.

Other information:
There is an easy workaround (also demonstrated in the attached file), which defers editing by connecting to Glib::signal_timeout() with a low priority.