The document modified status becomes set when I highlight part of the contents
of a cell with the mouse and type Control+C.  I first discovered this with
Control+C but other combinations such as Control+A for select all also set the
document modified status.  Interestingly, some combinations, such as Control+F
for find do not set the modified status.  This seems to indicate that there is
at least some routine which is inspecting key strokes and making a decision
about whether the cell has been altered or not.

The current behavior is wrong, since Copy does not modify the document.  It is
also annoying because I have many spreadsheets that are reference documents such
as a list of passwords.  I only open them to copy data out of and into other
applications.  Every time I open, copy, and close these documents I have to
respond to the document modified dialog asking if I want to save changes even
though no changes have been made.

Steps to reproduce (using attached spreadsheet test_modified_status.ods)
1) Select cell A1
2) Use the mouse to select the value 'a' in the cell input line above the
spreadsheet
3) Type Control+c to copy the value
4) Select cell A2 simply to move the focus away from the current cell.  The
document modified status will change.