Hi Jacek,
TestComplete interacts with standard Windows controls using Windows messages. Specifically, to open a combo box's drop-down list, TestComplete sends the
CB_SHOWDROPDOWN message to the combo box. After that, TestComplete uses the
CB_GETDROPPEDSTATE message to determine whether the list has actually dropped down.
Most likely, the issue occurs because your customized TComboBox doesn't react to these messages properly. So, please check with the application developers whether the combo box supports these messages.
