Data changed on mouseover even though data hasn't actually changed

Nobody has claimed this yet.

Take a seat, because this is a weird one.

Reproduction case: https://gist.github.com/bheisler/cdc7af6256c74e6e40d181248ebeab21
Druid version: Current master branch
Platform: Windows (I don't _think_ that matters for this one, but because it's so weird I'm not sure)

Running this program brings up a window. Mousing over the window results in a stream of messages to stdout saying that a change was detected; these are printed by the ChangeDetectedController in response to `old_data.same(data)` returning false. That is to say, there's an update caused by every window event even though (as far as I can tell) nothing in this code can actually change the data.

Expected behavior: The data should not be changed by mousing over the window.

Now, let me dig into why this is weird. This doesn't _look_ like a minimal case, but it is. Just about every further reduction or change I can think of causes it to stop happening. A non-exhaustive list of things that cause this behavior to stop:
* Changing the inner type of UniqueId
* Removing the ThingId::Dummy variant (which is _not used in this code_)
* Changing the contents of the ThingId::Unique variant
* Removing or changing the type of _any_ of Thing's fields, even though they also are not used in this code, except to instantiate a Thing.
* Not adding a Thing to the ThingEditor::things Vector
* Changing the type of ThingEditor::things
* Replacing the List with a Label

I've attached a debugger and found that the `Vector::ptr_eq` check in the Data impl for Vector is returning false, even though the _contents_ of the vector are exactly the same.

I have _no idea_ why this is happening, but I minimized this case out of my recipe manager app - it is happening in my actual app, and I can't find a way to make it stop happening with my real code without removing real functionality. My app uses `old_data.same(new_data)` to trigger an auto-save, so this is actually causing me a real problem.

Run the linked gist on the current master branch and observe the mouseover output from ChangeDetectedController. Inspect the Data implementation for Vector, especially Vector::ptr_eq, and trace why old_data.same(data) reports a change; done means mouseover no longer reports changes when the vector contents are unchanged.

Written by the indexing model from the issue text.