No `external` in Ubuntu 20.04 applications

I've cloned this repo, navigated into `webview-examples` and run `cargo run --example todo`.

Expected: TODO app runs and works
Actual: app starts, but no UI elements are displayed in the app

Seems like there's no `external` (`window.external`) object in the page:

On the other hand, `minimal` example works just fine, displaying a Wiki page.
Also both examples work on macOS and Windows for me.

I'm using stock Ubuntu with a few packages installed to successfully compile the examples.
```
lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 20.04.2 LTS
Release: 20.04
Codename: focal
```

Is there any way to fix it or at least do debug it further?