Make GtkPaned's handle larger visually by default
Discussed in the matrix room, so opening an issue so it doesn't get forgotten.
Despite paned handles having a reasonably large invisible drag area, what we actually see of the handle is a 1px width line which is very similar to non resizeable separators used for split headerbars (w/ sidebar). As such it isn't a very large click target (although the actual drag area is larger, but it is invisible) and there's not much way to know if a particular separator is a paned handle or not. There is a way for apps to indicate they want a wider handle, but I don't think that needing all apps to opt-in to not have those issues is a good way of doing things. The handle shouldn't have those issues by default ideally, so I think making it wider by default is a good idea.
The CSS that handles this in libadwaita is here.
ping @bertob, to decide what exact width to use for the handles.