itch.io

I think you need to send the event to slider1 in a slightly different way:

slider1.event["change" val]

There's another post on the forums explaining more here.

And you'll also need to define what val is in this context. That could be in either script.

For now I put both changes in button script:

on click do
 slider1.value:slider1.value - 1
 val: slider1.value
 slider1.event["change" val]
end