The only way to preview an article after source editing is by switching to visual editor, which is a bit inconvenient.
Topic on 2017 wikitext editor/Feedback
It does but hidden away (kind of) in the Publish changes button which upon clicking brings up a modal which is where the "Show preview" and "Show changes" resides in. This "issue" was already raised before couple of times by various editors including me as well but so far no solution has been pushed out yet. Alternatively, for now, you can use "Alt + Shift + P" for preview and "Alt + Shift + V" for show changes.
While having a easier working text box and better tools is nice, it's not worth having no solid preview/changes tab. The shortcuts work but not completely. Unlike before when you could view your preview while editing, it now takes you to a separate page where you can't edit, pretty annoying. Will be sticking with the old wikitext.