Jory,
thank you for you answer.
I have found out that part which is called /* EXPERIMENTAL */ on the line 53 in off-canvas-sidebars-frontend.class.php if enabled somehow “fixes” some parts. However .css which is fundamental for plugin to work interfere with original template .css. For example Vantage from SiteOrign has “sticky menu” which works fine but after activating the plugin it does not stick anymore.
I will try to find which part of code is stoping sticky menu and than let you know.