column width in % and manual resize of the same column are contradictory
because column loses its % width the moment you resize it directly.
All you can do is: turn the column into fixed width after grid initialization so that it can be resized manually thenceforth.
complete: function(){
var Cols = this.Columns();
Cols.alter(function(){
Cols.each(function(col){
col.width = col._width; //make it fix width.
col.resizable = true;
})
})
}