I also gave it a try, and couldn't get it to work. I imagine it has something to do with CodeMirror (the text editor) keeping the markdown and html aligned.
I proved that the rendered html can have smooth, but only if you include the HTML and CSS in the note itself. Not really useful.
Paste this into a sample note in Joplin to see what I mean:
Sample Note Code
# testing smooth scrolling, it works! (but only if you embed the html, so not actually useful)
<html>
<style>body {
background-color: #333;
display: flex;
align-items: center;
justify-content: center;
flex-direction: column;
}
nav {
background-color: #000;
display: block;
padding: 1em 0;
text-align: center;
width: 200px;
}
nav a {
color: #fff;
margin: 0 1em;
text-decoration: none;
}
.scrolling-box {
background-color: #eaeaea;
display: block;
height: 200px;
overflow-y: scroll;
scroll-behavior: smooth;
text-align: center;
width: 200px;
}
section {
display: flex;
align-items: center;
justify-content: center;
height: 100%;
}</style>
<nav>
<a href="#1">1</a>
<a href="#2">2</a>
<a href="#3">3</a>
</nav>
<div class="scrolling-box">
<section id="1">This is the first section</section>
<section id="2">This is the second section</section>
<section id="3">This is the third section</section>
</div>
</nav>