Thanks for pointing out mapping protocol access.  That's what I was (poorly) trying to describe in the __getitem__ docstring in v1.  It's a really concept.

I worked on the tests.  I added some more code to the classes, so I uploaded that too (sorry, but I needed help understanding the functions).

Testing your Page class seemed pretty easy, so hopefully I did it right.  I have added more methods to the class though, which I didn't add tests for yet, in case it's not the right direction.

I changed the existing test to use the Changes class.  I believe I did what you intended, but I didn't expand it to keys and highlights just in case.

Now, my difficulty.  I had trouble trying to figure out the test for 'save_as', so I wanted to show you what I had so far.  You don't need to fill it out; I just needed to see if I was on the right track.  Writing a test without code was difficult since it involved other classes.  So, I copied the existing code into the class to see how it would fit.  Observations:
1.  The `testcfg` was a IdleUserConfParser dictionary.  I made a dummy config parser to override Save instead of mocking it.  I think you prefer this way of doing that.
2.  Page needed to know it's 'config_type' so that it (or Changes) could reach into idleConf.  self.main on its own didn't know which part of the  config files to update.
3.  Following #2, I created a 'save' module within Page.  My options were idleConf.userCfg[page.name] (in Changes) or idleConf.userCfg[self.name] (in Page).  Based on your LoD comment, I thought Page should know about that and not Changes.
4.  Split out `save_as` into another method called `changed`, also in Page.  I thought it made sense for a Page to say if it's changed.
5.  set_user_value is the same as before, although I changed the docstring and it's in Page for the page.name vs self.name reason.  However, this still looks really clunky to me.

Questions:
1.  Any interest on making Page inherit from defaultdict to simplify additem?
2.  I'm not sure about the name for Page, but I haven't come up with an alternate.  My reason is that in configdialog, all the create functions are create_page_* and the names on the tabs don't match the config (eg, there's a General tab stored in main).  Unless you made that connection on purpose?  I was thinking of `page` more as a GUI item, maybe because of terms like webpage.  Maybe the solution is to make the functions create_tab_*?

I still haven't looked at extensions.  I should have more time tomorrow.

One note, I found a typo in the current test file.  In tearDownModule, it sets idleConf.userCfg to testcfg, but I think the intent was to set it back to the saved value, which is userCfg.

Thanks!