Sets 32 x 32 pixel retina drop down image for MacOS targets when your using old Xojo's or other such were you cannot use the new Image Set mechanism.
This property is not visible in the designer. You need to set it from code.
Do not use this property if you are using new style Image sets. If you have new style Image sets then simply assign Image set that contains 1x, 2x and 3x images to the DropDownIcon property.
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This property does nothing if the DropDownIcon property is nil.