Firefox Uncaught Error: Expected node to have parent

[mocanew]

Repro: https://jsfiddle.net/jmd4a2Lc/

console.log('start exec');

const outer = document.createElement('div');
document.body.append(outer);

const inner = document.createElement('div');
outer.append(inner);
outer.remove();

console.log('finished exec');

This code triggers a "Uncaught Error: Expected node to have parent" in Firefox (tested on versions 103 and 104). This does not happen on Chrome (105).

The problematic part seems to be the successive sync append and remove operations. The mutation observer (incorrectly?) emits a MutationRecord for the removed node.

[mocanew]

Added a potential fix in #55.
The first two conditions may be redundant as document.documentElement.parentNode should never be null. https://dom.spec.whatwg.org/#document-element:

The document element of a document whose parent is that document, if it exists; otherwise null.