[reactive-element] [lit-next] Appended multiple times within the same microtask causes createRenderRoot get called each time

[TimLuq]

I accidentally stumbled upon this scenario just now. If appended to the DOM multiple times before performeUpdate() has been able to execute, createRenderRoot() will be called each time. This may be unexpected behavior for subclasses overriding createRenderRoot(). An effect of this is when adoptable stylesheets are not supported the style tags are also duplicated multiple times (otherwise the stylesheets are readopted).

I would assume this is unwanted behavior and is probably a vestigial check from a time when createRenderRoot() and firstUpdated(changed)/_$didUpdate(changed) may have had different tasks to perform.

https://github.com/Polymer/lit-html/blob/f0d683b071aad02c423e45ea0d3db8816259759b/packages/reactive-element/src/reactive-element.ts#L735-L739

I propose changing the condition from !this.hasUpdated to !this.renderRoot. I am open to creating a PR for this (or for any other solution that the maintainers may prefer).

[sorvell]

Thanks for reporting this. This is just an oversight and and obvious bug. It should be a simple fix, thanks for the PR offer but we should be able to handle this quickly.

[sorvell]

Fixed via #1686.