When the SDK says that combo box style CBS_SIMPLE:

Displays the list box at all times. The current selection in the list box is displayed in the edit control.

this does not tell the whole story.

If you create such a combo box with a height of (for example) 20 pixels, then it will look just like an edit control and the user will be able to type in the control. Then if you add strings to the listbox they will not be visible because although the listbox is "displayed at all times" it has a Z-order behind the edit control. Despite the listbox being invisible, the user can select one of the listbox strings by using the up and down arrows, and then the string will appear in the edit control. As you can see from this, a CBS_SIMPLE combobox which is 20 pixels high is not much use.

The CBS_SIMPLE combobox comes to life if you dynamically change its height and Z-order using SetWindowPos. That way you can give the whole control "size" or more specifically "height". This makes the dropdown appear below the edit control on the screen. So for example if you and add 3 strings to the listbox, and use SetWindowPos to change the height of the combo box from 20 pixels to 80 pixels, the edit control part of the combo box will retain its original appearance and there will be a listbox shown as hanging below the edit control on the screen and containing the 3 strings. If you add a fourth string then a scrollbar appears in the listbox provided you have specified WS_VSCROLL when creating the combobox.

In this way you can cause the listbox to appear and to disappear dynamically. For example, you might want to make the listbox appear in response to user input to the edit control (reported to the combo box parent by CBN_EDITCHANGE), and to disappear on user selection of one of the strings (reported to the combo box parent by CBN_SELCHANGE). When causing the listbox to appear and disappear by changing the height of the combo box using SetWindowPos in this way, you have to pay attention to the Z-order and to redraw. You need to make sure the combo control is at the top of the Z-order when it is such a height as to cause the listbox to appear, and at the bottom of the Z-order when it is not. The control should be invalidated to ensure the background is redrawn when the listbox disappears.