AjaxControlToolkit ComboBox DropDown hiding on Scrollbar click

I use a AjaxControlToolkit Combobox with the following Settings:

SuggestAppend = SuggestAppend

DropDownStyle = DropDown

My DropDown height is limited, so I enabled vertical Scrolling via CSS. But if I click in the TextBox to open my DropDown and then click on the scrollbar, the DropDown immediately disappears.

This seems to be caused by the internal _onTextBoxBlur javascript event of the Combobox assembly.

Can I get rid of this behaviour without recompiling a modified assembly?

Any help would be much appreciated!