Stopping your Tumblr sidebar from scrolling requires a small edit to your blog theme's HTML code. Because every Tumblr theme is different, the steps for making this change vary from one theme to another. Although it may take a little trial and error, you should be able to make the change yourself without any advanced knowledge of HTML coding.
Click "Customize" on your dashboard, and then click "Edit HTML" on the Customize page to view your theme's HTML coding.
Press "Ctrl-F" to bring up the search tool, and then enter "