Hi Tran,
Thank you for contacting Syncfusion support.
We have checked the reported behavior of MultiSelectionComboBox in Windows 7 and we could reproduce the reported issue. While checking the reported issue, we found that the same issue has been reproduced in MS ListBox which has been used as drop down for our MultiSelectionComboBox control. In Windows 7, scrolling of ListBox has been working only when an item is selected in it and the same behavior has been working in MultiSelectionComboBox too. We have prepared video for your reference and it can be downloaded from the below location.
As a workaround, we suggest you to select an item before scrolling the drop down of MultiSelectionComboBox.
Regards,
Keerthana J