From RAD Studio API Documentation
TComboBox = class(TCustomComboBox)
class PASCALIMPLEMENTATION TComboBox : public TCustomComboBox
TComboBox combines an edit box with a scrollable list.
A TComboBox component is an edit box with a scrollable drop-down list attached to it. Users can select an item from the list or type directly into the edit box.
Note: The width of the button in a TComboBox is equal to the width that Windows uses for scrollbars. This width depends on the color scheme that the user has chosen (by right-clicking the desktop, selecting Properties, and choosing Appearance). On Windows XP, the user can specifically change the width of the scrollbar by clicking the Advanced button and selecting scrollbar as the Item. If you carefully size your combo box so all the text is visible, then a user running under a different color scheme may find that the text is obscured by the button. One solution is to set the scrollbar wide when developing the application. Most color schemes use a scrollbar that is at most 21 pixels wide.