Backbone route broken, but no consistent way to reproduce

This is a race condition: in D7 Edit, the Create.js PropertyEditor widgets are being loaded conditionally/JIT, and visiting a URL with the #edit fragment causes EditApp to be initialized before the metadata callback — along with the necessary PropertyEditor Widgets JS — has been loaded.

However, for a reason I don't understand yet, I can no longer reproduce this after clearing the browser cache.

This should be tracked down via git bisect.

After clearing the browser cache, I stopped being able to reproduce this. I *think* the trick to start reproducing it again, is by:
1. enable edit mode
2. browse to a different URL
3. use the browser's back button
4. now consistently reproducible

So it somehow seems to become not reproducible due to strange browser cache interactions, causing the execution order to be different…?

Today I again can't reproduce this anymore. So weird.

If this pops up again, I'm just going to remove the Backbone route; it's no longer in D8 core right now either, and it's better to have them more in sync rather than spend extra time debugging this in D7.

Let's not let this increase Edit's bug count.

This will be irrelevant soon, because I'm working on backporting the Drupal 8 implementation of in-place editing, which does not use Backbone Routes.