How to catch leaflet event and send it to a Traits app?

Question

I have a Python Traits app which consists in a instance of the HTML class (to say short).

In this instance, I have a leaflet map.

That works fine (it displays a leaflet map).

I know how to catch keyboard shortcut in leaflet, for instance:

document.addEventListener('keydown',
function(event)
{if (event.ctrlKey && event.key === 's')
{alert('Ctrl + s pressed!');}});

That works fine.

What I want to do is catching leaflet event and send it to my Traits app.

I tried something like this:

document.addEventListener('keydown', function(event) {
if (event.ctrlKey && event.key === 's') {
var event = new CustomEvent('ctrl_s_pressed');
document.dispatchEvent(event);}});

for the leaflet part,

and

ctrl_s_pressed = Event

def _ctrl_s_pressed_fired(self):
        print('Ctrl-s pressed')

for the Python/Traits part.

But this does not work: no message displayed from my Traits app.

Any hint?

TIA