This happens because the live preview feature uses the ApiParse module, which parses the content model as-is. mirage-navigation is a MediaWiki message and has the wikitext content model; therefore it will be interpreted as wikitext.
Ideally, a hook would allow overriding in ApiParse.
| Status | Subtype | Assigned | Task | ||
|---|---|---|---|---|---|
| Open | BUG REPORT | None | T304355 Live preview doesn't work with the preview of Mediawiki:mirage-navigation | ||
| Open | Feature | None | T136907 Live preview for "Preview page with this template" | ||
| Resolved | Anomie | T65523 Provide an API for template sandbox |
Not using live preview is a workaround. Implementing support in Mirage would require hacking or redesigning livepreview in a slimmed-down version. The preview is easily substituted thanks to it being available as a mustache template, but that would be a lot of work for a rarely used feature.
Marking as lowest accordingly.