Preview template for node with custom template? I have a custom content type, 'Article with Header' for which I have a custom twig template, 'page--article-with-header.html.twig', which manages the layout for this content type. When I preview the node before publishing, the template used is page.html.twig. I would like to have the same layout for previewing as for when the node is published, but I don't see any options for doing this. The template suggestions that come up in preview mode (page--node--preview.html, for example) would affect every content type.

Is there a naming convention for preview mode twig templates for custom content types that I'm missing? Is there a preprocessing function I can implement that will force the preview mode for this content type to use the correct template?
The naming convention is
node--[type]--[viewmode].html.twig

So in your case it would be

node--article-with-header--preview.html.twig

See https://www.drupal.org/docs/8/theming/twig/twig-template-naming-conventions
for more info.