Plumb an event through to content/ that is sent just before 'onload'.

Plumb an event through to content/ that is sent just before 'onload'.

This is to support a change for Chrome packaged app windows that helps to avoid
a flash of white when loading a page. Specifically, the app delays showing the
window until the 'onload' event has been fired. However, many things depend on
the page being visible at 'onload' time, so this change sends a message to the
content layer before the 'onload' event is sent, so that any messages generated
by JavaScript during the 'onload' event will arrive at the embedder after the
window has been shown.

The new event simply mirrors the 'didHandleOnloadEvents' path, but notifies
before the onload event is sent.

BUG=225843

[eseidel, 2013-04-10 19:47:25]

On the surface it's not obvious to me how this is different from a capturing
event listener for "onload" attached to the document.

[eseidel, 2013-04-10 19:48:01]

Nate may be sad for more Loader callbacks. :)

[jeremya, 2013-04-10 19:48:15]

On 2013/04/10 19:47:25, Eric Seidel (Google) wrote:
> On the surface it's not obvious to me how this is different from a capturing
> event listener for "onload" attached to the document.

Could you point me at some code that I could read to learn more about that? I
didn't realise that was even a thing :)

[eseidel, 2013-04-10 19:50:06]

https://developer.mozilla.org/en-US/docs/DOM/EventTarget.addEventListener ?

document.addEventListener("load", myAwesomeListener, true)?

[eseidel, 2013-04-10 19:52:14]

Maybe you want to show the window at DOMContentLoaded?  I guess that could be
long before onload fires.  Your goal I assume is to show the window just when
the document is ready, but not before?

onload doesn't necessarily mean an app is ready of course. :)  Many web
applications just start getting going at onload.

[jeremya, 2013-04-10 19:52:25]

On 2013/04/10 19:50:06, Eric Seidel (Google) wrote:
> https://developer.mozilla.org/en-US/docs/DOM/EventTarget.addEventListener ?
> 
> document.addEventListener("load", myAwesomeListener, true)?

Ah, we need the event in C++ in Chrome-land. This doesn't expose any new events
to JavaScript.

[jeremya, 2013-04-10 19:53:19]

On 2013/04/10 19:52:14, Eric Seidel (Google) wrote:
> Maybe you want to show the window at DOMContentLoaded?  I guess that could be
> long before onload fires.  Your goal I assume is to show the window just when
> the document is ready, but not before?
> 
> onload doesn't necessarily mean an app is ready of course. :)  Many web
> applications just start getting going at onload.

DOMContentLoaded is too early, apps still flash white if I hook that to window
show.

My goal is to avoid the flash of white.

[jeremya, 2013-04-10 19:54:11]

On 2013/04/10 19:53:19, jeremya wrote:
> On 2013/04/10 19:52:14, Eric Seidel (Google) wrote:
> > Maybe you want to show the window at DOMContentLoaded?  I guess that could
be
> > long before onload fires.  Your goal I assume is to show the window just
when
> > the document is ready, but not before?
> > 
> > onload doesn't necessarily mean an app is ready of course. :)  Many web
> > applications just start getting going at onload.
> 
> DOMContentLoaded is too early, apps still flash white if I hook that to window
> show.
> 
> My goal is to avoid the flash of white.

(In my brief experimentation, listening to onload has been sufficient to at
least load the background color/image. I'd prefer to wait for first paint, but
that's hard because Reasons.)

[jeremya, 2013-04-11 18:29:35]

ping- any further thoughts on this?

[jamesr, 2013-04-11 20:52:55]

I don't think onload or DOMContentLoaded are very good proxies for "will this
flash white".  That's a graphical question which has only a passing relationship
with DOM loading.  Some pages are ready to display well before DOMContentLoaded
and some aren't ready to display until well after onload.  While it may
correlate well on some set of manual tests, it's not a good general solution. 
If you want to know something about rendering or graphics you should ask the
rendering or graphics systems.  The loader definitely shouldn't be involved.

Can you file a bug describing precisely when you want to receive a notification?
 We can discuss there how to wire that up.

[jeremya, 2013-04-11 22:25:22]

On 2013/04/11 20:52:55, jamesr wrote:
> I don't think onload or DOMContentLoaded are very good proxies for "will this
> flash white".  That's a graphical question which has only a passing
relationship
> with DOM loading.  Some pages are ready to display well before
DOMContentLoaded
> and some aren't ready to display until well after onload.  While it may
> correlate well on some set of manual tests, it's not a good general solution. 
> If you want to know something about rendering or graphics you should ask the
> rendering or graphics systems.  The loader definitely shouldn't be involved.
> 
> Can you file a bug describing precisely when you want to receive a
notification?
>  We can discuss there how to wire that up.

Done: http://crbug.com/230634

Closing this review.