(I've accidentally linked to this task in a few unrelated revisions - if you're here from no.wikipedia.org please see T153579 instead)

This was also reported by @geraki at https://www.mediawiki.org/wiki/Topic:Tht3r0lic3wqzko6

This is the API call we'll need to use:

{
	"action": "parse",
	"page": "Page being previewed",
	"templatesandboxtitle": "Template:Template being edited",
	"templatesandboxtext": "New wikitext of template being edited"
}

Reported again as https://www.mediawiki.org/wiki/Topic:U8rxdhfkh5w0zohg

Is anything happening here? I have made a lot of unnecessary template edits already because of the lack of a preview function. If it is not introduced, I guess template pages should either use the CodeEditor or the old wikitext mode per default , otherwise template editing will never properly work.

Change 570984 had a related patch set uploaded (by BrandonXLF; owner: BrandonXLF):
[mediawiki/extensions/TemplateSandbox@master] Support page preview in the 2017 wikitext editor

https://gerrit.wikimedia.org/r/570984