Is there any documentation for building/customizing custom UI controls? I need to customize an existing out-of-box control (OpportunityCoverageUserPersonPicker.control), but there does not seem to be any GUI or documentation on what any of the properties do.
More specifically, I am trying to make the select popup of OpportunityCoverageUserPersonPicker.control (CoverageUserCustomPicker.screen) launch with a specific where clause, but no properties I have tried work (I have tried selectPopupWhere and selectPopupSearchWhere, not knowing if either of these properties are even valid due to lack of documentation handy).