#112019 included open/closed state of the parent panel in socket
visibility calculation. This prevents dragging links, but also disables
other features that should still work, such as drawing links.
A narrower condition is needed for icon visibility vs. general socket
visibility. The cases which use the new `is_icon_visible` condition:
- Drawing socket selection outlines (same as unselected sockets)
- Drawing multi-input sockets (same as unselected sockets)
- `node_find_indicated_socket`, used by a wide range of mouse click
operators, including the link-drag operator that was cause for
#112019.
Cases using the original `is_visible` (true even if parent panel is
collapsed):
- `nodeLinkIsHidden` draws links only when at least one socket is
visible.
- `node_update_basis`, sockets still added to layout even if icon isn't
rendered.
- `node_update_hidden`, panels are ignored for "hidden" nodes, all
sockets are rendered.
- `NODE_OT_link_make` operator for finding "best" sockets to connect.
- `node_link_viewer` finding sockets to connect to a viewer node.
- `get_main_socket` used for insert-on-links (find sockets to splice
into) and some shader previews
- `node_gather_link_searches`, suggestions for adding a new node at the
end of a link.
Pull Request: https://projects.blender.org/blender/blender/pulls/112326
5d77d8d832