Fix issue two scrollbars and exclude download button from sidebar height

What I have done

• I fixed the issue with two scrollbars. Now, when the sidebar is open, the main scrollbar will disappear
• Now the download button is excluded from the height of the sidebar.
  Before: when the sections overlap with the download button, it didnt show a scrollbar, eventhough parts arent visible.
  Now: as soon as the sections overlap with the download button, it will show the necessary scrollbar
• Minor change: I removed the rounded edges from the download button