Scrollbar not showing, but panel does

I have a table surrounded by an with the a 开发者_JS百科css class:

DIV.scrollableTable {
 overflow-y: scroll;
 height: 100%; 
 width: 100%;  
}

when I load the page, the scroll panel is present, but the bar isn't. And in order for me to scroll I have to select everything in the table and move the mouse down. Any idea what the problem could be?

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In order for the scroll bar to show, the content of DIV.scrollTable must be higher than "DIV.scrollTable".