intended use of scrollbar […]
and when the scrollbar appears, it may cover a part of the text when text lines are long
Do I understand you correctly, that you think the intended behavior of scrollbars is to hide content ? 
I would say a scrollbar should generally* not cover the content below it. A scrollable area should work like a table with two columns: content in one column, scollbar in the other. If the scrollbar is AUTOMATIC then sometimes the second column has width=0. (this seemed to be the case for GTK2, btw.)
*exceptions are of course overlay scrollbar.
Below is a concrete example. The user searches for kanji. Each letter is added to the search history on the right (which is a vbox inside a GtkScrolledWindow)
