(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

Change 570984 merged by jenkins-bot:

[mediawiki/extensions/TemplateSandbox@master] Support page preview in the 2017 wikitext editor

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

More styles should be added to improve the appearance, and wikipage.content hook should be fired.

In T153367#7137447, @Tranve wrote:

More styles should be added to improve the appearance, and wikipage.content hook should be fired.

What's wrong with the appearance that needs improvement?

In T153367#7139776, @BrandonXLF wrote:

In T153367#7137447, @Tranve wrote:

More styles should be added to improve the appearance, and wikipage.content hook should be fired.

What's wrong with the appearance that needs improvement?

IMHO, parser output should be wrapped with mw-body and mw-body-content class to import appearance, and by saying "and wikipage.content hook should be fired" I mean if this hook isn't fired, some gadgets like Reference Tooltips won't function.

UPD: I think the TitleInputWidget should also set $overlay: true so the suggestion area would be wider.

You can use $overlay: this.$overlay within a dialog.

Categories should also be visible in the preview because someone may want to test correct categorization using a template.