On Apr 30, 2015, at 8:17 PM, Hayato Ito <hay...@chromium.org> wrote:
> On Fri, May 1, 2015 at 2:59 AM Ryosuke Niwa <rn...@apple.com> wrote:
>> > On Apr 30, 2015, at 6:00 AM, Domenic Denicola <d...@domenic.me> wrote:
>> >
>> >> This essentially forces distribution to happen since you can observe the 
>> >> result of distribution this way. Same with element.offsetWidth etc. And 
>> >> that's not necessarily problematic,
>> >
>> > OK. So the claim that the current spec cannot be interoperably implemented 
>> > is false? (Not that I am a huge fan of <content select>, but I want to 
>> > make sure we have our arguments against it lined up and on solid footing.)
>> >
>> >> but it is problematic if you want to do an imperative API as I tried to 
>> >> explain in the bit you did not quote back.
>> >
>> > Sure, let's dig in to that claim now. Again, this is mostly clarifying 
>> > probing.
>> >
>> > Let's say we had an imperative API. As far as I understand from the gist, 
>> > one of the problems is when do we invoke the distributedCallback. If we 
>> > use MutationObserve time, then inconsistent states can be observed, etc.
>> >
>> > Why can't we say that this distributedCallback must be invoked at the same 
>> > time that the current spec updates the distribution result? Since it 
>> > sounds like there is no interop problem with this timing, I don't 
>> > understand why this wouldn't be an option.
>> There will be an interop problem. Consider a following example:
> The return value of (2) is the same in either case. There is no observable 
> difference. No interop issue.
> Please file a bug for the spec with a concrete example if you can find a 
> observable difference due to the lazy-evaluation of the distribution.

The problem isn't so much that the current shadow DOM specification has an 
interop issue because what we're talking here, as the thread title clearly 
communicates, is the imperative API for node distribution, which doesn't exist 
in the current specification.

In particular, invoking user code at the timing specified in section 3.4 which 
states "if any condition which affects the distribution result changes, the 
distribution result must be updated before any use of the distribution result" 
introduces a new interoperability issue because "before any use of the 
distribution result" is implementation dependent.  e.g. element.offsetTop may 
or not may use the distribution result depending on UA.  Furthermore, it's 
undesirable to precisely spec this since doing so will impose a serious 
limitation on what UAs could optimize in the future.

- R. Niwa

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