Clearly specify initUIEvent

[dtapuska]

Since initUIEvent probably isn't going away anytime soon. It should be clearly documented with respect to its arguments instead of its current specification of no arguments.

It is present in multiple browsers according to MDN; https://developer.mozilla.org/en-US/docs/Web/API/UIEvent/initUIEvent

We need to decide what the actual specified behavior is:

Chromium has this:
void initUIEvent([Default=Undefined] optional DOMString type,
[Default=Undefined] optional boolean bubbles,
[Default=Undefined] optional boolean cancelable,
[Default=Undefined] optional Window? view,
[Default=Undefined] optional long detail);

But it identifies that all the values should not be optional. We need to figure out what other vendors have.

@garykac @foolip @smaug----

[foolip]

In this issue I would include all the init*Event methods in the spec, especially initMouseEvent and initKeyboardEvent. Even if the arguments differ in different implementations, there's probably some number of leading arguments that would be enough to support.

[foolip]

Context: https://bugs.chromium.org/p/chromium/issues/detail?id=692720

[smaug----]

How does chromium handle those undefined values? Does undefined boolean end up treated as false?

[foolip]

Yes, Chromium's IDL is effectively this:

void initUIEvent(optional DOMString type = "",
                 optional boolean bubbles = false,
                 optional boolean cancelable = false,
                 optional Window? view = null,
                 optional long detail = 0);

As in whatwg/dom#387, I think we should make the first argument non-optional and make the rest optional with defaults.

[annevk]

Yes, this should be aligned with DOM.