Does the 'On Select' action for Combo Boxes and other controls run once when selecting an item in the drop-down or each time you select an item?
Yes On Change runs each time an item in a dropdown is selected and is the better property to use for Dropdowns. However OnSelect runs every time the control is selected
OnChange will fire each time you select or deselect an item.
OnSelect will fire as soon as you click on the control, irrespective of whether you select or deselect anything.
Have you distinguished these two properties?
Actually, the OnSelect triggers every time you select the control.
The OnChange property triggers every time you select an item that differs from your previously selected item.
In addition, I want to remind you that the Text input controls differs from the Combo Box on OnChange property. The OnChange property of the Text input only triggers when you input a completely different strings from your previously input and you should click anywhere outside the Text input control.
Hope it could help you at some degree.