From Xojo Documentation

Property (As Boolean )
aComboBox.AllowFocusRing = newBooleanValue
BooleanValue = aComboBox.AllowFocusRing

New in 2019r2

Supported for all project types and targets.

If True, the control indicates that it has the focus with a ring around its border; if False, the appearance of the control does not change when it has the focus. At present, the focus ring appears only on macOS and Linux. On all platforms, the combobox gets a text insertion point when it has the focus. The default is True.

Sample Code

The following code turns off AllowFocusRing.

ComboBox1.AllowFocusRing = False