The SGH_CLICK command indicates that the user has clicked the select
button of the mouse within the gadget select box.  There are no parameters
following the SGH_CLICK command longword.

Intuition will have already calculated the mouse position character cell
and placed that value in sgwork.bufferpos.  the previous bufferpos value
remains in the sgwork.stringinfo.bufferpos.

Intuition will again use the sgwork fields listed above for sgh_key.  that
is, the workbuffer, numchars, bufferpos and longint fields values may be
modified by the hook and are used by Intuition if sga_use is set when the
hook returns.