[Bug 19763] [Custom]: Consider copying event listeners when upgrading elements

https://www.w3.org/Bugs/Public/show_bug.cgi?id=19763

Daniel Buchner <danieljb2@gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |danieljb2@gmail.com

--- Comment #1 from Daniel Buchner <danieljb2@gmail.com> ---
I would be against copying event listeners for a few reasons, here's why:



- there are a number of things set by user JS after the page is parsed that
cloning a node does not copy, to only do a subset would be confusing.

- by copying event listeners, which will likely contain calls to the custom
element's unique prototype methods, we create a situation where events may be
able to be invoked by the interface end-user *before the element it upgraded*.
This means that the refs to anything custom about the element would fail. It is
better to say to the developer "assign all user-defined JS events not part of
the component's operation itself, inside of the upgrade event (as opposed to
DOMContentLoaded,Load, etc)"

- I cannot be sure of this, but I would bet performance would be impacted
negatively if we allowed this (though I am unsure to what extent)

Bottom Line: push developers to do all such operations within the upgrade event
loop, it is a more cohesive and expected position than hopscotching on what we
will copy and what we won't (child elements and originally-specified HTML
source attributes withstanding, for obvious reasons)

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