def on_new(self, view) has no window

 view.window() is unsafe in a few situations. As a workaround you can use sublime.active_window() in most cases.

Well, that does not solve the fact that view.window() *is* unsafe.

Thank-you FichteFoll, however it would appear that doesn't solve my problem as the new tab still appears to the immediate right of the currently focused tab. I guess this is due to the tab (view) not having being attached to a window at the point at which the event fires.

Ok, I see what you mean. I agree with you that this should be possible in the on_new command and that those functions should return something (values should be assigned). But maybe this just wasn't intended for the function so maybe an "on_post_new" callback should be added.

Well, for the time being you can use this workaround with sublime.set_timeout:

import sublime
import sublime_plugin

class EventDump(sublime_plugin.EventListener):
  def on_new(self, view):
  self.view = view
  sublime.set_timeout(self.on_post_new, 200)

  def on_post_new(self):
  view = self.view
  w = view.window() or sublime.active_window()
  print view.window().__class__, w.__class__
  grp = w.active_group()
  print w.get_view_index(view)
  print grp.__class__, w.views_in_group(grp)
  w.set_view_index(view, grp, len(w.views_in_group(grp)) - 1)

Brilliant, thank-you for that!

You can also try to define on_load as well and use that to assign the group index after defining a value or something in on_new so this does not trigger more often than it should.