Unable to access input sources for inline WebXR session

Question

I would like to access input sources - in particular, the Quest touch controllers - during an inline WebXR session (not entering immersive VR mode; I am not using three.js or A-Frame) on the Quest browser. I request and store the session with the code:

let xrSession;

navigator.xr.requestSession("inline").then( function(session) {
    xrSession = session;
});

The variable xrSession does in fact store an XRSession type variable with an inputSources field, but inspecting inputSources via the Javascript console, it is always an empty array; I am unable to access any data.

How can I access input sources in WebXR, or the Quest controller data (such as button pressed states) in general, in the Quest browser without entering VR mode?