When writing code for owner-draw combo boxes try to keep these guidelines:
Use SetItemData because it easy to get it with the DRAWITEMSTRUCT without making any obsolete and time consuming function calls.
Check the state and action of the item to know when to draw selection rectangles. Check for the state ODS_SELECTED, and ODA_SELECT to handle this case.
Find a way to know when the control has focus, and make the control show it. The focus is better passed as a parameter to the drawing functions. This will be a better logic. Come up with a different logic if you have some different types of handling them.
Remember to return the old device context settings if you have changed them.If you don’t weird things might happen. This is one of the most important things to be done as advised by Microsoft.
Try to avoid copy-pasting code when writing combo and list boxes with similar interface. Implement a single class which can be called for drawing the interface.