File generated from file watcher not appearing as "child" of original "source"

Hello,

I am creating my own command line file watcher, (OS X 10.12.1, Xcode 8.1, in C++) and have a basic system up and running. All works as expected, except that when the file watcher triggers in the IDE ...

(IntelliJ IDEA 2016.3
Build #IU-163.7743.44, built on November 17, 2016
JRE: 1.8.0_112-release-408-b2 x86_64
JVM: OpenJDK 64-Bit Server VM by JetBrains s.r.o)

... the generated file is not added to the file browser a child of its parent. 

(File with .rtest extension is the watched file, file with same name and .html extension is the one generated by the watcher.)

But other files, e.g.: CSS files from SCSS files, display fine:

Is there some setting/preference somewhere that I need to be configuring?

I have tried creating the output file from stdout, as well as creating the file from directly within the watcher, both show the same behaviour.

Any hints greatly appreciated.

Charlie