Could you add a setting for this? Chrome for Android doesn't support extensions and I really don't want screen space taken up by a persistent toolbar.

An easy way to make it a setting would be to use a CSS variable for the position:

::root {
    --interaction-bar-position: fixed;
}

.container .article-actions.relative {
    --interaction-bar-position: relative;
}

.container .article-actions {
    ...
    position: var(--interaction-bar-position);
}

And then a checkbox in settings that would allow the user to change the setting.