I had assumed that it was built in to Scintilla since [that docs page I mentioned before](http://www.scintilla.org/ScintillaDoc.html#MultipleSelectionAndVirtualSpace) [1] seems to be about Scintilla only and it explicitly mentions `Ctrl` in conjunction with the mouse. I had also tried `Ctrl-click` in SciTE and that worked as expected. After a cursory `grep` through the latest Scintilla and SciTE, I didn't find `Ctrl-click` logic in either of them. There's some kind of autogenerated interface in SciTE which makes things like this hard to search for there, though. I did stumble across the logic for the selection adding behaviour bound to `Ctrl-D` in Sublime Text. Altogether that suggests to me that the `Ctrl-click` behaviour is probably in SciTE, not built-in`to Scintilla.
So I've now tried calling `SCI_ADDSELECTION` on `Ctrl-click`, in the same place I removed the code from, but it still doesn't seem to do anything. I've confirmed it makes it to the right part of the big switch statement in the local copy of Scintilla though.
[Here's the diff](https://gist.github.com/Ryan1729/2244914f0ac7856fed42f8379fb4cd36).
[1] Regarding the link, I meant to do something [`like this`](http://www.scintilla.org/ScintillaDoc.html#MultipleSelectionAndVirtualSpace). I've put the backticks inside the square brackets this time. I knew there was a way that it worked.