Union selection selects the entire canvas when no selection

[PabloRufianJimenez]

Description If you use any selection tool in Union mode with an empty selection, the entire canvas is selected. I'm not sure whether this is an intentional feature, but I think it's more intuitive if, with an empty selection, union mode worked like replace mode.

To Reproduce

1. Have a canvas with no selection
2. Pick a selection tool (e.g. Rectangle Selection)
3. Set Selection Mode to "Union"
4. Do any selection
5. The entire canvas will be selected

Additional Info

Version Linux Mint 22.1 Pinta 3.1

[cameronwhite]

I agree that I'd expect union to behave like replace in this case.

I suspect the underlying issue is that there isn't currently a clear distinction in the code between having nothing selected (which implicitly acts like everything is selected for some purposes, like when an effect is clipped to the current selection), and actually having the whole document explicitly selected. Right now the only difference is the visibility flag for whether the selection is drawn