At this moment, the column width auto-fit doesn't take the column titles and the active areas into account.
However, you should be able to use the "width" property of a column to set a minimum width (reflecting the title and/or active areas - they are known in advance). If you turn the shrinking off, this value will be used as the starting value when extending the width:
rowHeaderColumns: [
{ text: "Column1 ", width: 60 },
// ...
},
rowHeaderWidthAutoFit: true,
rowHeaderWidthAutoFitShrink: false,