While e.g. the `simpleviewer` and `singlepageviewer` examples work, since they're based on the `BaseViewer`-class, the standalone `pageviewer` example currently doesn't support either XFA- or StructTree-layers. This seems like an obvious oversight, which can be easily addressed simply by exporting the necessary functionality through `pdf_viewer.component.js`, similar to the existing Text/Annotation-layers.
While working on, and testing, these changes I happened to notice a number of smaller things that's also fixed in this patch:
- Ensure that `XfaLayerBuilder.render` always have a *consistent* return type, to prevent possible run-time failures in `PDFPageView`; PR 13908 follow-up.
- Change the order of the options in the `XfaLayerBuilder`-constructor to agree with the parameter order in the `DefaultXfaLayerFactory.createXfaLayerBuilder`-method.
- Add a missing `textHighlighterFactory`-option, in the JSDocs for the `PDFPageView`-class.
- A couple of small tweaks in the `TextLayerBuilder.render`-method: Re-use an existing Array rather than creating a new one, and replace an `if` with optional chaining instead.
*Please note:* For now XFA-support is currently disabled by default, similar to the regular viewer.
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