Why does this work:

document.body?.appendChild(renderer.domElement)

While this doesn't?

val target = document.getElementById("right_pane")?
target?.appendChild(renderer.domElement)

document.body? is an Element and document.body? is an HTML element.... not sure why there are two but there has to be a converter or something... or someway to make this work. Elements seems to allow appending a string as HTML via innerHTML while HTMLElements seem to do something with Nodes... don't really get this, feels like object interfaces aren't formalized.

HTMLElement is just a subtype of element. By “doesn’t work”, do you mean it doesn’t compile (you have an extra “?” on the first line) or it has no effect?

Yeah, it compiles but with unintended results. I actually figured it out as soon as you messaged me, lol. the missing word was "apply" and then it appears I had access to the two things I needed to append and html element string and also a child node

document.getElementById("right_pane")?.apply {
    appendElement("div") {
        innerHTML = ProjectionToolbar().view
    }
    appendChild(viewer.renderer.domElement)
}