The pagebanner extension doesn't seem to work with the api=parse and api=expandtemplates endpoints. This affects all clients that use these mediawiki API endpoints.
See the output of http://pagebanner.wmflabs.org/w/api.php?action=expandtemplates&text={{PAGEBANNER:Toledo_banner_Puerta_de_Alfonso_VI.jpg}}
Compare against the output of http://pagebanner.wmflabs.org/w/api.php?action=expandtemplates&text={{uc:Toledo_banner_Puerta_de_Alfonso_VI.jpg}}
The expandtemplates API endpoint should be able to return the wikitext corresponding to the execution of a transclusion / parser function.
Similarly the parse API endpoint should be able to return the parsed HTML corresponding to the wikitext.
But, see http://pagebanner.wmflabs.org/w/api.php?action=parse&text={{PAGEBANNER:Toledo_banner_Puerta_de_Alfonso_VI.jpg}}
Again, compare with http://pagebanner.wmflabs.org/w/api.php?action=parse&text={{uc:Toledo_banner_Puerta_de_Alfonso_VI.jpg}}
Specifically, Parsoid (as one client of the mediwiki API) uses the api=expandtemplates endpoint to support transclusions (parser functions and templates) -- it uses the endpoint to fetch the expanded wikitext for the transclusions. This has 2 impacts:
- Parsoid's rendering of these pages will be different from the PHP parser rendering
- Editing of these pages in visualeditor will be affected -- it will not corrupt those pages, but the banner will not show up and you may not be able to edit these transclusions.