FireFox 51 (curet) fails Custom Elements: `Element#attachShadow` was not patched. about custom-elements

[bicknellr]

Oh, this is just telling you that Element.prototype.attachShadow doesn't exist (you're running in a situation without Shadow DOM) and that it didn't patch the (non-existent) function. Maybe I should have used console.info instead of console.warn for these. You shouldn't need to worry about this if you aren't using Shadow DOM.

[puppetmaster3]

More info:

• the component code is here, so you don't have to view code: view-source:https://www.masons-foundation.org/_compHost/home-card.html

• safari works.

• firefox dev preview v53 has same error

• firefox does work on a different page https://amp.masons-foundation.org/post/asites/

But https://www.masons-foundation.org/home/feed/ in FF makes error:
Custom Elements: Element#attachShadow was not patched.

[bicknellr]

If you're using a Shadow DOM polyfill (like Shady DOM) and you're still seeing this error, you need to load the Shadow DOM polyfill first so that it can do its patching before the custom elements polyfill does its own.

edit: Additionally, the custom elements polyfill needs to use the Shadow DOM polyfill's methods to inspect / walk the tree. If it doesn't, the custom element polyfill will be operating on the 'composed' tree and callbacks can end up being called on the wrong elements and at the wrong times.

[bicknellr]

@puppetmaster3, I'm going to close this since I don't think this is actually a bug. If you find that this still happens in situations where you're using Shadow DOM, please leave a comment and I'll reopen it.