Where should I download the schema definition for BXML files? It'd be great to have for IDEs to help with autocompletion.
Sadly there is no schema (neither XSD nor DTD) for BXML. The problem is that BXML is too much dynamic format. The reason is well described in this issue: https://issues.apache.org/jira/browse/PIVOT-385
The best approach would be to have an IDE plugin but again sadly there is no one - at least not for IDEA and Eclipse.
There is an open issue from 2014 to create generator of XSD for BXML validation during of build (Maven, Gradle, ...) but without any further progress: https://issues.apache.org/jira/browse/PIVOT-936
It is the same case as for FXML used in JavaFX where there is also no schema and it is also impossible to have it valid, complete and deterministic because of the same reasons. Happily in this case there are IDE plugins for it - at least for IntelliJ IDEA.