Add a floating button to switch compact mode when using touch screen

[Elecleus]

Just as the title says, add a button at the corner of the browser window when the compact mode is disabled.
This would greatly improve the user experience on touch screens.

The button should be draggable as it may cover something.

[mr-cheffy]

This?

[Elecleus]

No. This one locates at the panel, and what I want is floating on the web page.
But the functions are exactly the same.

Like this:

It should appear at one of the corners, and be able to be dragged to another corner.

And when the compact mode is disabled, it should be hidden.