If think I should have mentioned it, is use the falcon starter theme, with its is_blockcart module.
The modal block get triggered by this function :
https://github.com/Oksydan/is_shoppingcart/blob/731b7ba31ab7e05747c98b4c197046384886cd3e/_theme_dev/src/js/theme/index.js#L74
Then I guess I should find a way to trigger this
prestashop.on('updateCart')
event ? (do you think this is possible without updating the cart content ?)
nothing happens with :
onclick="prestashop.updateCart"
or copy and adapt the whole function...