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Re: [webcomponents] Adjusting offsetParent, offsetTop, offsetLeft properties in Shadow DOM

From: Scott Miles <sjmiles@google.com>
Date: Sat, 23 Mar 2013 21:50:03 -0700
Message-ID: <CAHbmOLYXJTjQYAV=5g0qEZXhJvtvVRkzHhhzj0UN=ogsOKO19w@mail.gmail.com>
To: Dominic Cooney <dominicc@chromium.org>
Cc: WebApps WG <public-webapps@w3.org>
Sorry for the late response, this is one of those bad cases where agreement
was expressed as silence.

This is a thorny problem, but my initial reaction is that you threaded the
needle appropriately. I don't see how we avoid some lossiness in this


On Mon, Mar 18, 2013 at 1:48 AM, Dominic Cooney <dominicc@chromium.org>wrote:

> Summary: I think the Shadow DOM spec should specify how offset* properties
> are handled around shadows. Further, I think "traversable" and
> "non-traversable" shadows should be handled uniformly. The offsetParent
> property should return the first offsetParent at the same level of shadow
> as the receiver, or the document, to maintain lower-boundary encapsulation.
> And the offset{Top, Left} properties should be accumulated across "skipped"
> offsetParents.
> Problem:
> It seems the consensus is that there will be two kinds of shadows, ones
> that are exposed to the page through properties such as
> HTMLElement.shadowRoot, and ones that aren't [1]. The language is emerging
> but for now I will refer to these as "traversable" and "non-traversable"
> shadows respectively.
> In both cases, there's a question of how to handle HTMLElement.offset*
>  properties, particularly offsetParent. [2]
> Let's talk about a specific example:
> <div id="a">
>   <div id="b">
> {#a's ShadowRoot}
>   <div id="c" style="position: relative; left: 10px;">
>     <div id="d">
>       <content>
> In this case, the positioned ancestor of #b is #c. What should the result
> of b.offsetParent be?
> If the ShadowRoot is "not traversable" it is clear that b.offsetParent
> should NOT be c. If it were, it would be very difficult to use
> not-traversable shadows that don't accidentally leak an internal node.
> (Especially when you consider that c could be a pseudo-element, and the
> author could set position: relative on the element that way.)
> Discussion:
> I think the offset{Parent, Top, Left} properties should be adjusted. This
> means that in the above example, b.offsetParent would be <body> and
> b.offsetLeft would be silently adjusted to accumulate an offset of 10px
> from c. I think this makes sense because typical uses of offsetParent and
> offsetLeft, etc. are used to calculate the position of one element in the
> coordinate space of another element, and adjusting these properties to work
> this way will mean code that naively implements this use case will continue
> to work.
> This behavior is unfortunately slightly lossy: If the author had #c and
> wanted to calculate the position of #b in the coordinate space of #c, they
> will need to do some calculation to work it out "via" body. But presumably
> a script of this nature is aware of the existence of Shadow DOM.
> The question of what to do for offset* properties across a shadow boundary
> when the shadow *is* traversable is a vexing one. In this case there is no
> node disclosed that you could not find anyway using .shadowRoot, etc. tree
> walking. From that point of view it seems acceptable for offsetParent to
> return an offsetParent inside the (traversable) shadow.
> On the other hand, this violates the lower-boundary encapsulation of the
> Shadow DOM spec. This means that pages that are using traversable shadows,
> but relying on convention (ie "don't use new properties like .shadowRoot")
> to get the encapsulation benefits of Shadow DOM, now have to audit the
> offsetParent property. It also means you need to have two ways of dealing
> with offsetParent in both user agents and author scripts. So for simplicity
> and consistency I think it makes sense to treat both traversable and
> non-traversable shadows uniformly.
> Dominic
> [1] Thread starts here: <
> http://lists.w3.org/Archives/Public/public-webapps/2013JanMar/0535.html>
> [2] <http://www.w3.org/TR/cssom-view/#offset-attributes>
> <http://goto.google.com/dc-email-sla>
Received on Sunday, 24 March 2013 04:50:31 UTC

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