Menu button is jumpy when scrolling up/down on mobile

Hi everyone!

I’m trying to get my menu button to stick to the bottom of the screen nicely, but when scrolled on mobile (at least Safari) the button jumps up and down in an ugly way as it responds to the browser’s address bar appearing/disappearing.

I know it has something to do with the Nav bar and Nav button both having fixed positions.

Anyone have good ideas about how to fix a menu button to the bottom of the page without any twitchy movements?

Here: ☀️Wed Design Galleria☀️ Nettisivut yrityksellesi | you can see the difference in behaviour between the blue fellow on the bottom-right and the menu button on the bottom-left. I like the smooth movement more.

All is fine on desktop and tablet because there the menu button gets it’s position from the top of the screen.

So please try this out on mobile to see what I’m talking about.

Here is my site Read-Only: [LINK][1]