On Thu, Apr 30, 2015 at 8:57 PM, Ryosuke Niwa <rn...@apple.com> wrote:

> ...
> >
> > 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.
element.offsetTop must use the distribution result, there's no way to know
what your style is without computing your distribution. This isn't any
different than getComputedStyle(...).color needing to flush style, or
getBoundingClientRect() needing to flush layout.

Distribution is about computing who your parent and siblings are in the box
tree, and where your should inherit your style from. Doing it lazy is not
going to be any worse in terms of interop than defining new properties that
depend on style.

- E

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