In the package editor it’s currently not possible to preview how the part looks on a specific layer.
I’d need a way to view only top/bottom layers. Ideally I could also turn on/off all layers separately, e.g. to view only copper or only documentation layers.
Can we put the layer selector in the package editor too?
Yes, that should be possible. I just wonder how it should be integrated into the library editor. In the board editor it is integrated as a dock widget, stacked with other dock widgets. But in the library editor we don’t have dock widgets, but a fixed region used for the metadata widgets. Adding a dock widget would probably be too crowded and might look a bit strange 
I think a dock widget (“Metadata” and “Layers”) would be fine.
Hmm yeah I think it would require some refactoring to convert the “Metadata” widget to a dock widget (dock widgets are window-global, but the metadata is separate for each tab, so it must be replaced when switching between the tabs). At least this would automatically fix https://github.com/LibrePCB/LibrePCB/issues/359 
Feel free to open an issue for this task 
Issue opened: