Dock the Administration block.
"Open" it and close all its "categories" (Users, Courses, Grades, Competencies...) using the disclosure triangle.
"Close" the block.
Resize the browser window height in order to have enough room to display the Admin block as it is now.
"Open" the block once again and also open all its "categories" using the disclosure triangle.
The vertical scroll bar is not activated when the heigh of the block exceed the height of the window.
Rolling the mouse ball, I get the scroll of the page and not of the content of the block.