Очень хорошо работает нижеследующее:

declare long GetWindowLong in Win32API integer, integer
declare integer WindowFromPoint in Win32API integer, integer

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*
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FUNCTION Get_ComboBoxState
LPARAMETERS toObject

LOCAL llState, nTop, nLeft, nBottom

nTop = ObjToDeskTop(toObject, 1)
nLeft = ObjToDeskTop(toObject, 2)
nBottom = ObjToDeskTop(toObject, 4)

llState = .T.
*** GetWindowLong(hwnd, -16) retrieves the window styles.
IF BITTEST(GetWindowLong(WindowFromPoint(nLeft + 5, nBottom + 5), -16) ,31) OR ;
BITTEST(GetWindowLong(WindowFromPoint(nLeft + 5, nTop - 5), -16), 31)
*** Если в определенных точках (немного слева и сверху ComboBox или немного слева и снизу ComboBox) - раскрытый список -
*** WindowStyle = MS_POPUP или WS_POPUP Or WS_BORDER Or WS_SYSMENU
*** WAIT WINDOW NOWAIT 'Раскрыт...' + TIME()
llState = .T.
ELSE
*** WAIT WINDOW NOWAIT 'Закрыт...' + TIME()
llState = .F.
ENDIF


*********************
*
*********************
FUNCTION ObjToDeskTop
LPARAMETERS toObject, tnPosition

LOCAL nTop, nLeft, nRight, nBottom, lIsTitleBar
STORE .NULL. TO nTop, nLeft, nRight, nBottom

IF TYPE("_Screen.ActiveForm") == "O" AND NOT IsNull(_Screen.ActiveForm) AND PCOUNT() > 1 AND TYPE("tnPosition") == "N"
IF VAL(_VFP.Version) >= 8.0
lIsTitleBar = _Screen.ActiveForm.TitleBar = 1
ELSE
lIsTitleBar = NOT (LEN(_Screen.ActiveForm.Caption) = 0 AND _Screen.ActiveForm.Closable = .F. AND _Screen.ActiveForm.ControlBox = .F. AND _Screen.ActiveForm.MaxButton = .F. AND _Screen.ActiveForm.MinButton = .F. AND _Screen.ActiveForm.Movable = .F.)
ENDIF

DO CASE
CASE tnPosition = 1
IF _Screen.ActiveForm.WindowType = 0 && Host form is Modeless
nTop = IIF(_Screen.ActiveForm.HalfHeightCaption, SYSMETRIC(34), IIF(lIsTitleBar, SYSMETRIC(9), 0)) + ;
IIF(_Screen.ActiveForm.BorderStyle = 3, SYSMETRIC(4), SYSMETRIC(13)) + ;
IIF(_Screen.ActiveForm.ShowWindow = 2, _Screen.ActiveForm.Top, OBJTOCLIENT(_Screen.ActiveForm, 1)) + 3 + ;
ObjToClient(toObject, 1) && if there is a menu: + SYSMETRIC(20)
ELSE && Host form is Modal
nTop = IIF(_Screen.ActiveForm.HalfHeightCaption, SYSMETRIC(34), IIF(lIsTitleBar, SYSMETRIC(9), 0)) + ;
IIF(_Screen.ActiveForm.BorderStyle = 3, SYSMETRIC(4), SYSMETRIC(13)) + ;
OBJTOCLIENT(_Screen.ActiveForm, 1) + 3 + ;
ObjToClient(toObject, 1) && if there is a menu: + SYSMETRIC(20)
ENDIF
CASE tnPosition = 2
IF _Screen.ActiveForm.WindowType = 0 && Host form is Modeless
nLeft = IIF(_Screen.ActiveForm.BorderStyle = 3, SYSMETRIC(3), SYSMETRIC(12)) + ;
IIF(_Screen.ActiveForm.ShowWindow = 2, _Screen.ActiveForm.Left, OBJTOCLIENT(_Screen.ActiveForm, 2)) + ;
ObjToClient(toObject, 2) && Left
ELSE && Host form is Modal
nLeft = IIF(_Screen.ActiveForm.BorderStyle = 3, SYSMETRIC(3), SYSMETRIC(12)) + ;
OBJTOCLIENT(_Screen.ActiveForm, 2) + ;
ObjToClient(toObject, 2) && Left
ENDIF
CASE tnPosition = 3
nLeft = ObjToDeskTop(toObject, 2)
nRight = nLeft + ObjToClient(toObject, 3) && Left + Width
OTHERWISE
nTop = ObjToDeskTop(toObject, 1)
nBottom = nTop + ObjToClient(toObject, 4)
ENDCASE
ENDIF

RETURN IIF(NOT TYPE("tnPosition") == "N", .NULL, ;
IIF(tnPosition = 1, nTop, ;
IIF(tnPosition = 2, nLeft, ;
IIF(tnPosition = 3, nRight, nBottom))))


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* cboF - класс на основе comboBox, можно поместить в контейнер
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*********
* DropDown
*********
IF TYPE("This.Parent.tmrComboBoxState") == "O"
This.Parent.tmrComboBoxState.Interval = 50
This.Parent.tmrComboBoxState.Enabled = .T.
ENDIF

***************
* DropUp - Создаем
***************
This.ReadOnly = .T. && Если список закрыт, то всегда так !
This.ZOrder(1) && to Back - не обязательно!


********************************
tmrComboBoxState - Объект класса Timer, тоже в вышеуказанный контейнер!
********************************
******
* Timer
******
*** WAIT WINDOW NOWAIT "Отслеживаю ReadOnly " + TIME()
IF This.Parent.cboF.ReadOnly = .F.
*** WAIT WINDOW NOWAIT "Проверка состояния " + TIME()
IF Get_ComboBoxState(This.Parent.cboF) = .F. && Список закрыт (закрылся !)
This.Parent.tmrComboBoxState.Enabled = .F.
This.Parent.tmrComboBoxState.Interval = 0 && Выключили (Пусть будет так - правой рукой за левое ухо - почему то иногда This дает объект cboF, а не tmrComboBoxState)
This.Parent.cboF.DropUp() && Установили cboF.ReadOnly = .T. и задвинули на задний план !
ENDIF
ENDIF


*** cboF.ReadOnly = .T. - установить
*** Как раскрыть -
контейнер.KeyPress()
контейнер.MouseUp()