Two-way binding observers not initialised in some circumstances

[Rich-Harris]

Another wild bug courtesy of @TehShrike. Repro here.

Essentially what happens is this:

• The oncreate function is successfully called for each <Top> component, and component.get('top') is correct
• Next comes bindings. The first binding is created (for the final component, because the order shouldn't really matter here), and that causes component._set(...) to happen (component being App.html in this case), which triggers an update of the main fragment, which causes top to become undefined for the remaining <Top> components, because their bindings haven't been initialised yet so idToTop is incomplete
• By the time the second and third bindings are initialised, the differs check returns false, because the value of top is undefined, which means the observer is not initialised.

So somehow we need to prevent the _set from causing the update. If we can figure that out, then we'll also be saving some unnecessary work from happening (the top-level component could update just once, instead of three times).

Answers on a postcard...

[TehShrike]

Should there be a separate issue created for the binding-in-an-array-with-a-key-when-the-keys-change issue?

On that topic, remembering this thing from chat, I tried doing this to work around that issue, but it didn't seem to help.

[Rich-Harris]

@TehShrike not 100% sure I understand what that issue is, but it sounds like the answer is 'probably' 😀

[TehShrike]

That was the "I need to use keyed list iteration" issue - I'll open a different issue for the issue I found with keyed blocks.