Sidebar menu: how can I create a sidebar menu that pops up (slowly) at the left side

You can try:

https://community.thunkable.com/t/free-extension-sidebar-navigation-update-12-04-2018/5376

or if it doesn't have to be a sidebar:

https://ai2.metricrat.co.uk/guides/who-needs-a-sidebar-anyway

or have a look through the extensions list to see if there is something for you: