How does 'Open with' work internally?

[phw]

If it is a Qt5 app the best way would be if the application would be patched to support the FileOpen event, because that’s what RefsReceived is triggering.

The alternative is to set the QT:QPA_FLAGS attribute for the application to Q_REF_TO_ARGV, the Qt will attempt to convert the initial RefsReceived message into command line arguments.

See also my comment on

CudaText 64bit: Text/Code Editor: RC. Please report issues

There are two ways to handle that. On Haiku when a file should be opened by an application a RefsReceived message is being sent to this application. Haiku’s Qt implementation by default translates this into a Qt FileOpen event. If an application supports this everything works out of the box as it should. But since the FileOpen event is otherwise only being used on macOS many Qt applications that were only written with Linux or Windows in mind don’t implement that event. So the ideal solution …