NSOutlineView: How does Cocoa determine a click in a cell can start a drag operation?

FROM : Stéphane Sudre
DATE : Sun Feb 03 13:55:06 2008

Question:
---------

How does Cocoa determine a click in a cell can start a drag operation?

And how can I tell it to allow a NSButtonCell can be clicked to start 
a drag operation?

Problem:
--------

I have a NSOutlineView with 3 columns. The first column has its data 
cell to a subclass of NSButtonCell to display a checkbox (with a label)

+--------------------+-----------+-----------+
| [ ] My checkbox    | My Label  | My Label  |
| [ ] My checkbox    | My Label  | My Label  |
+--------------------+-----------+-----------+

All the columns and data cells are set to be non editable.

The subclass of NSButtonCell overloads - (BOOL) 
trackMouse:inRect:ofView:untilMouseUp: so that the cell does not 
respond to clicks.

Now this doesn't seem to be enough as a click on a cell of the first 
column can not start a drag operation.

If I disabled the cell, a click can start the drag operation.

Current Investigation Results:
------------------------------

- Mailing list archives: nothing relevant

- Google: nothing obviously relevant

- GNUStep source code: Apparently, returning NO for - (BOOL) 
trackMouse:inRect:ofView:untilMouseUp: should be enough.

- Medium: overloading - (BOOL) trackMouse:inRect:ofView:untilMouseUp: 
is apparently enough for an Apple tool.

- Advanced Medium: Provided a potential solution involving an API 
from the NeXT world.