SUMMARY
I created a project containing a ConTeXt file structure (I chose the file ending .ctx for all files), and picked the correct file as the project’s master file. I updated the “ConTeXt” build tool to be `context --nonstopmode --silent=all %source”`. Trying to run it gives me “Could not determine the master file for this document”.

To work around this, I have to manually set the project’s master file as global master document using the Settings menu.

STEPS TO REPRODUCE
1. Create a project
2. Set the project’s master file to something with a non-.tex suffix (I assume that might be part of the problem)
3. Try to run a build tool with `%source` in it from any file in the project

OBSERVED RESULT
“Could not determine the master file for this document” in the output window

EXPECTED RESULT
The selected build tool is run on the project master file

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.17.5
KDE Frameworks Version: 5.66.0
Qt Version: 5.14.0