Hello,

with

BINDEVENT(Thisform.mygrid.col1.myheader,"MouseUp",Thisform.mygrid.columns(Thisform.mygrid.columncount).myheader,"MouseUp")

you're binding the MouseUp of the header in the first column to the new column's header.
I think you want it the other way.

BINDEVENT(Thisform.mygrid.columns(Thisform.mygrid.columncount).myheader,"MouseUp",Thisform.mygrid.col1.myheader,"MouseUp")

Regards
Christian