mynode/packages/app-desktop/gui/NoteEditor/utils/useEffectiveNoteId.ts

import { useEffect, useRef } from 'react';
import { NoteEditorProps } from './types';

export default function useEffectiveNoteId(props: NoteEditorProps) {
	// When a notebook is changed without any selected note,
	// no note is selected for a moment, and then a new note gets selected.
	// In this short transient period, the last displayed note id should temporarily
	// be used to prevent NoteEditor's body from being unmounted.
	// See https://github.com/laurent22/joplin/issues/6416
	// and https://github.com/laurent22/joplin/pull/6430 for details.

	const lastDisplayedNoteId = useRef<string>(null);
	const whenNoteIdIsTransientlyAbsent = !props.noteId && props.notes.length > 0;
	const effectiveNoteId = whenNoteIdIsTransientlyAbsent ? lastDisplayedNoteId.current : props.noteId;

	useEffect(() => {
		if (props.noteId) lastDisplayedNoteId.current = props.noteId;
	}, [props.noteId]);

	return effectiveNoteId;
}