The following test case should print "deleted" to indicate that we're not leaking memory.

let slint = require("./index");

const registry = new FinalizationRegistry(() => {
    console.log("deleted");
});

let m = slint.loadFile("./test.slint");
let app = new m.App();
app.test.setHandler(() => {
    console.log(app);
});

registry.register(app);

app = null;

global.gc();

Run inside api/napi, alongside a test.slint:

export component App inherits Window {
    callback test();
}

Run with node --expose-gc ./test.js

If you comment out the code in the closure, you'll see that app gets garbage collected.

Our theory is that the reference to the JS closure kept in the Rust implementation is seen as a root by the garbage collector. One approach we discussed as a remedy would be to store all the callback closures inside the JS object (as properties in an array perhaps), instead of using a strong napi::Ref that becomes a root.