Usually a default constructor is called on a pad if one is not explicitly stated. For most types this seems to be the case. Here is an example where this does not happen:
This also happens with Observer, BehaviorSubject, and I believe I saw it in custom datatypes as well a while ago. It would be useful if a constructor is always automatically called, this issue has led to some difficult-to-diagnose bugs.
Something odd seems to be happening with constructors. If I run the following patch, the renderer window refuses to show up. Once I clear the assignment of the MouseNotification constructor, the renderer window appears, albeit without a rectangle. And then once I re-assign it to Create, the rectangle finally appears.
All on build 369, by the way.