GridDefaultElement is an option for GridBox which specifies what to insert when a new element is created interactively in a GridBox.
The default setting for GridDefaultElement is "\[Placeholder]" or "".
When creating palettes, GridDefaultElement is typically set to ButtonBox[""].
The setting for GridDefaultElement is used to determine the contents of new columns or rows created with , or (Control-Return).
See Section 2.9.11.
New in Version 3.