How can we know that |window| should have on<x> attribute without a declaration in the IDL?

Suppose that there is:
    interface <x>Event : Event { ... };
How can we know that |window| should have on<x>?  I thought that not all events have their own corresponding on<x> on |window|.

Especially for window.onappinstalled, I don't see AppInstalledEvent defined in an IDL.
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/modules/app_banner/WindowInstallation.idl?q=appinstalled+file:%5C.idl$&sq=package:chromium&l=10

Could you elaborate your idea a little bit more (and optionally how feasible is it)?

In general, some interfaces derived from EventTarget may have on<x> and some other wouldn't.  So it's not trivial whether on<x> should be defined or not.

FYI, the spec defines GlobalEventHandlers interface and uses "implements GlobalEventHandlers" as necessary.
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/dom/GlobalEventHandlers.idl

True, I guess it would be bad to generate those if they aren't explicitly listed in some IDL somewhere.

Feel free to WontFix, this was just a suggestion. We could also have some kind of warning that says "you have an event type that doesn't have a corresponding on<x> attribute".

This came up because ... I'm having trouble finding a source, but someone at Mozilla said we shouldn't have had this bug in the first place, if Chrome worked like Firefox because apparently Firefox generates these automatically.

mgiuca@, do you know how FF generates them automatically?  Do they have an additional input?  For example, if we had the following input file, we could auto-generate them.

    # EventType, has onxxx on [Global]?
    <x>Event, true
    <y>Event, false
    ...

Or do they use an extended attribute to annotate it?  I guess that they should have something as an extra input...

#3 I've looked everywhere I can for a citation on that but it must've been mentioned to me in person (during the discussions we had around beforeinstallprompt). I'm not familiar with the Firefox codebase and it's possible I misunderstood. We can close this as a WontFix if you like.

Thanks for checking things.  I close this issue.  Feel free to reopen as necessary.