Download top-up invoices and review article previews more easily

Download a top-up invoice directly from its transaction detail page.

Payment method controls have more space around them. Article type labels make Rewrite Article results easier to recognize. Code blocks display correctly in article previews. Rewriting an article from a collection opens the appropriate preview. Discover-source selection is limited to one category to keep the choice clear. Initial loading of the landing page has been lightened.