[uievents] Is CompositionEvent supposed to have .locale? (#48)

Copied from W3C Bugzilla: https://www.w3.org/Bugs/Public/show_bug.cgi?id=28924

##### Olli Pettay 2015-07-07 17:24:39 UTC
It is not clear how CompositionEvent should behave per current draft.
Should it have .locale property which is then initialized for example using
the legacy initCompositionEvent?
And should the dictionary for ctor have locale too?

Some version of some spec (I'm getting lost with D3E, UIEvents and UIEventsExtensions) had .locale in KeyboardEvent.

Anyhow, since .locale is there for initCompositionEvent, I'll add it to the dictionary too in Gecko. Guess work here.

##### Comment 1 Olli Pettay 2015-07-07 17:27:53 UTC
Oh, CompositionEvent used to have readonly attribute DOMString? data, but now it is non-nullable. Why the backwards incompatible change?

##### Comment 2 Takayoshi Kochi 2015-07-08 08:14:06 UTC
I'm not following the D3E discussion lately, but ~2years ago when I
investigated this, only IE was implementing the .locale on Composition events.
It was hard to formulate the proper value for .locale, and I gave up
including it in IME API.

I'm not sure how popular the usage is, but I guess we should avoid
abandon .locale in the spec as it is poorly spec'ed and quite hard
or sometimes meanless to expose the value on some environment.

Note: You may wonder why .locale is filled for each event, even though
.locale should rarely change between events.  I heard that the rationale
was for not leaking user's environment without user's consent - typing on
keyboard is considered an affirmative action that user let the user agent
(or the author of the content) know in which locale he/she is typing text.

Note2: the intention to include .locale in IME API was not only to
expose user's typing environment, but to *change* the locale so that
an author can *force* which language is expected to type in for
some form element etc. (i.e., .locale is writable). It might sound
useful, but quite hard to implement interoperably.

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