Do not redefine class in customElement decorator if already defined

[tronicboy1]

Reference Issue

Fixes #3794

Change customElement decorator to not define a custom element if it is already in the registry.

This eliminates DOM exceptions when multiple scripts are accidentally imported.

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[justinfagnani]

Even though this is a small code change, it is a major behavior change that would require an RFC. @customElement() is only a declarative shorthand for customElements.define() - it's been important that the behavior matches. We also don't put features in decorators that are not available in plain JS.

Given a single global registry, and no way to tell if two registrations for the same name are compatible, throwing is the more correct and conservative thing to do. We may wish to explore some other option, but that would involve some way of element definition describing what other definitions they are compatible with and still throwing if they aren't known to be compatible.

[tronicboy1]

@justinfagnani
I totally understand where your coming from with that. You could have multiple packages trying to define the same name and have no way to see that as an error.

I'll close this PR, sorry for wasting your time!

[justinfagnani]

@tronicboy1 not a waste. I think we should do something here for use cases that need it, it's just a little complicated. Hopefully scoped element registries solve most cases.

[tronicboy1]

@justinfagnani
First time hearing about scoped registries. Your proposal makes a lot of sense and I for one would be very happy to see it realized!

thanks again for all of your work.