> On Apr 27, 2015, at 7:32 PM, Steve Orvell <sorv...@google.com> wrote:
>> Perhaps we need to make childrenChanged optionally get called when 
>> attributes of child nodes are changed just like the way you can configure 
>> mutation observers to optionally monitor attribute changes.
> Wow, let me summarize if I can. Let's say we have (a) a custom elements 
> synchronous callback `childrenChanged` that can see child adds/removes and 
> child attribute mutations, (b) the first option in the proposed api here 
> https://gist.github.com/rniwa/2f14588926e1a11c65d3, (c) user element code 
> that wires everything together correctly. Then, unless I am mistaken, we have 
> enough power to implement something like the currently spec'd declarative 
> `select` mechanism or the proposed `slot` mechanism without any change to 
> user's expectations about when information in the dom can be queried.

Right. The sticking point is that it's like re-introducing mutation events all 
over again if we don't do it carefully.

> Do the implementors think all of that is feasible?

I think something alone this line should be feasible to implement but the 
performance impact of firing so many events may warrant going back to 
micro-task timing and think of an alternative solution for the consistency.

> Possible corner case: If a <content> is added to a shadowRoot, this should 
> probably invalidate the distribution and redo everything. To maintain a 
> synchronous mental model, the <content> mutation in the shadowRoot subtree 
> needs to be seen synchronously. This is not possible with the tools mentioned 
> above, but it seems like a reasonable requirement that the shadowRoot author 
> can be aware of this change since the author is causing it to happen.

Alternatively, an insertion point could start empty, and the author could move 
stuff into it after running. We can also add `removeAll` on HTMLContentElement 
or 'resetDistribution' on ShadowRoot to remove all distributed nodes from a 
given insertion point or all insertion points associated with a shadow root.

- R. Niwa

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