Why is my application freezing when I pass the IntelliMouse pointer
over a DBLookupComboBox?
There is a conflict between the IntelliMouse driver v2.2d and TDBLookupComboBox. The error is reproduced by running an application which has a TDBLookupComboBox on a machine with IntelliMouse v2.2d. Click on the arrow in the box to show the dropdown list. Now move the mouse wheel, and the application freezes. The application must be killed to recover.
This issue does not occur with a TComboBox because TDBLookupComboBox is not a TComboBox descendent.
This issue was fixed in Delphi 5. Here is a workaround for Delphi 4 which requires adding two lines to DBCTRLS.PAS. The .DCU file will need to be rebuilt after adding these lines:
// This line takes care of the AV. ListLink.DataSet could be // nil after the WM_MOUSEWHEEL wasn't handled and the Intellimouse // driver decided to send a WM_VSCROLL procedure TDBLookupListBox.WMVScroll(var Message: TWMVScroll); begin SearchText := ''; (**** ADD THIS LINE ****) if ListLink.DataSet = nil then Exit; with Message, ListLink.DataSet do end; // This takes care of the "hang". The control is getting a // WM_KILLFOCUS immediately after the *second* WM_MOUSEWHEEL. // Focus is retained by the drop-down list, making it appear // that the application had stopped responding } procedure TDBLookupComboBox.CloseUp(Accept: Boolean); var Msg: TMsg; ListValue: Variant; begin if FListVisible then begin if GetCapture <> 0 then SendMessage(GetCapture, WM_CANCELMODE, 0, 0); (**** ADD THIS LINE ****) SetFocus; ListValue := FDataList.KeyValue; ... end;
Last Modified: 28-DEC-00