Custom controls and libraries

The page and the [abap2UI5](artifact:abap2ui5-abap2ui5) service have to share an origin: an `endpoint` that resolves to another origin is refused, and abap2UI5 rejects a POST whose `Origin` names another host. A host that routes by the URL hash, such as a Fiori elements app or an app with a UI5 router, needs abap2UI5 1.146.0 or later, because 1.145.0 clears the host's hash after every roundtrip. One frontend per page: the first control that starts decides which endpoint the frontend comes from.

Needs Standard ABAP 7.50 or higher, and an abap2UI5 whose frontend reserves the resourceRoot `z2ui5_cci`: a frontend predating that rename reserves `z2ui5ccc` and cannot resolve the controls. Frontend artefacts only one company needs, such as an in-house reuse library, a corporate icon font or company CSS, are what [abap2UI5-addons/custom-controls-customer](https://github.com/abap2UI5-addons/custom-controls-customer) is for, the same mechanism under the second reserved root `z2ui5_ccc`. ExportSpreadsheet needs SAPUI5: under OpenUI5 `sap.ui.export` is missing and the button renders disabled.