What was done in 3.4:

• use the new DialogDefinitionRegistry
• but keep registering old-style compatibility dialogs (rely on auto-conversion for the new Pages app).

This maintained status quo in the compatibility Pages app (+/- BLOSSOM-269, fixed in 3.4.1).

This introduced support for simple dialog/tab-factories in new Pages app.
However there, there are still certain objects that the old BlossomFormDialogPresenter used to make injectable, mostly Vaadin 7 Item, JCR Node and node path. These use cases must keep using the compatibility Pages app for the time being. Magnolia 6 UI Framework does not have configurable form-dialog presenters, nor Vaadin 7 Items.