Reopening - I don't think it is complete yet:
Do:

a = num2cell (rand (2));

then right-click any cell, select "clear" => sets it to numeric 0, while I'd expect to see empty (0x0 matrix).
Also IMO the Del key should be usable for emptying cell array cells or struct fields (works Matlab), but OK that may be a feature request.

In the above example, assigning a char value to a cell with previous numeric value works for any cell. However, if/once a cell contains a char value, in the VE it can only be assigned another char value.
IOW, the VE doesn't understand that leading quote is required to indicate a numeric value and a missing leading quote indicates numeric or maybe logical value.
In Matlab, char value are shown decorated with quotes. Maybe it is better if Octave would do the same - IIRC the VE as it was last year behaved like that.

(hg id a979bb895b64 avoid reinterpret_cast ...)

I pushed the following changeset:

  http://hg.savannah.gnu.org/hgweb/octave/rev/ea058ec5ef30

Now the concept of data rows/columns and display rows/columns are managed seprately and the size of the display is always kept larger than the size of the data to allow adding elements beyond the existing limits of a matrix.

This change doesn't cause the display to be resized when scrolling beyond the current display size, but that feature should be fairly easy to add.

Empty cells or struct fields cannot be assigned a value, the display '[0x0 matrix]' seems immutable (that is, I haven't found a way to assign it a value in the V.E.).
Matlab allows editing of such cells so they can be assigned a value.

The other way round, editing a cell + emptying it merely sets it to empty string (''), pressing the Del key does not work (in the latter case the terminal shows "syntax error" and a popup appears "failed to evaluate...").
Right-clicking the cell and selecting 'clear' sets it to numeric 0, Matlab doesn't have that option.

For cell arrays and struct fields, Matlab consistently sets the cell contents to empty '[]' (an empty double matrix, checked by inspecting the class of the cell value), irrespective of the original cell value class.