The instance editor popup should be embedded somewhere and simply link to whatever the active instance is.

Additionally, the scrollbar should stay in the same place in the case of changing to a difference instance of the same type; this makes it easier to make multiple instances when modifying the same variable for each.

As for other things that would make the instance editor easier to use:
* Don't automatically clear unused instances on compile. Maybe move it to Clean Compile, or a separate command. Show that an instance is unused by graying it out, italicizing the name, and/or drawing some kind of warning icon over it.
* A button for Duplicate Active Instance. Maybe buttons for the "Generate ..." context menu commands too.
* Variables should be in declaration order, not alphabetically. Or, let us reorder them by dragging and dropping their names or a handle. Maybe shown under collapsible sections according to the type they're declared in, e.g. "tag" would be under a "datum" header. The headers would probably be best in reverse order by default; i.e. the type is first, the parent_type is second, and so on.
* Option to hide variables starting with an underscore (or by regex); since there's no "private" modifier.