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Re: Imperative API for Node Distribution in Shadow DOM (Revisited)

From: Ryosuke Niwa <rniwa@apple.com>
Date: Thu, 30 Apr 2015 10:55:59 -0700
Cc: Anne van Kesteren <annevk@annevk.nl>, Justin Fagnani <justinfagnani@google.com>, WebApps WG <public-webapps@w3.org>, Erik Bryn <erik@erikbryn.com>, Dimitri Glazkov <dglazkov@google.com>, Edward O'Connor <eoconnor@apple.com>, Steve Orvell <sorvell@google.com>
Message-id: <3C064916-7BA9-4E5E-86A8-202A282459B2@apple.com>
To: Domenic Denicola <d@domenic.me>

> 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:

```js
someNode = ~
myButton.appendChild(someNode); // (1)
absolutelyPositionElement.offsetTop; // (2)
```

Now suppose absolutelyPositionElement.offsetTop is a some element that's in a disjoint subtree of the document. Heck, it could even in a separate iframe. In some UAs, (2) will trigger style resolution and update of the layout. Because UAs can't tell redistribution of myButton can affect (2), such UAs will update the distribution per spec text that says "the distribution result must be updated before any _use_ of the distribution result".

Yet in other UAs, `offsetTop` may have been cached and UA might be smart enough to detect that (1) doesn't affect the result of `absolutelyPositionElement.offsetTop` because they're in a different parts of the tree and they're independent for the purpose of style resolution and layout. In such UAs, (2) does not trigger redistribution because it does not use the distribution result in order to compute this value.

In general, there are thousands of other DOM and CSS OM APIs that may or may not _use_ the distribution result depending on implementations.

- R. Niwa
Received on Thursday, 30 April 2015 17:56:30 UTC

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