Fast scroll button is hidden underneath bottom toolbar

[theFireProject]

Actual behaviour

If you scroll to the bottom in a long document, the fast scroll button is hidden underneath the bottom toolbar:

Expected behaviour

The fast scroll buttun should always be visible. It should start from the top of the area where the pdf document is displayed and go to the bottom of it. The probleme is, the way that the scroll button has to go changes when the bottom toolbar is hidden and the display area becomes bigger. Also the position of the scroll button has to move to be still correct whenever the bottom toolbar appears/disappears. This movement should be a fluent animation and not a jump.