Tooltip for red box

Hey, I am using gSender on a RPi3 and connecting remotely from my mac.
This works great!

In the interface there is a red square with rounded corners.
Seems like I can click on it, but I have no idea what it does.
If adjust feed speed it moves…

Please add a description in a tooltip or in the documentation what this is, does and indicates.

It should be appearing as a slider.


What browser are you accessing it from.

Firefox 116.0.2
Mac OS 12.6.8

nb. I am using this with a 3018 CNC. Here is the entire screen: