Just to be absolutely clear what we're talking about, here's a gif recording of an unconfigured SciTE 4.0.2:
In the gif I `Ctrl-click` twice (unfortunately you can't see my mouse,) to make two additional selections, perform some edits, then simply left click to go back to a single selection.
I guess I was assuming Scintila handled more of the GUI, rather than, as appears to be the case, just managing the editor state.
@AdamDanischewski Those seem like good suggestions. assuming extra selections are actually working, just not showing in the UI, SCI_ROTATESELECTION should allow a quick demonstration that will actually show up. I'll get started on that.