Properties linking to Commons pages (P373 Commons category, P935 Commons gallery) should use word completion when they are edited. Possibly, "Commons page" can be defined as a new datatype in properties.
In addition, some intelligent function could scan wiki pages linked by interwikis from the item for {{commonscat}}, {{commons}} and similar templates and propose their parameters as prioritized values of the edited property.
This bug is not about commons-MEDIA datatype but about possible commons-PAGE "datatype" (linking generally to pages of other projects, not only to media files).
Technically speaking, this does not pertain to the Wikibase extension, since we use the 'string' datatype for such properties.
Perhaps we should go with a gadget.
Ahhhh now I understand it. ThanksRicordisamoa.
In general I agree with you that this should probably (at least initially) be a gadget. I am however also hesitant to add more scripts to item pages when we're right now trying really hard to make them load faster. So I guess it'd have to be a non-default gadget. Do you agree?
Actually, every autocompletion feature has to be written in JavaScript, even if shipped with the extension.
But I was arguing that the extension _cannot_know_ if the string is used as a page title or e.g. as an authority control identifier.
A consequence of the messy Commons-Wikidata linking system.