maby use mouse_ent and plase a invisible entity over the panel
i dont know a other way

mouse_ent
This pointer gives the entity the mouse pointer is over (if any).

Type:
Entity*, read-only
Example:
function maus_entkill()
{
if(mouse_left == 1) // left Mousebutton pressed?
{
if(mouse_ent)
{
ent_remove(mouse_ent); // remove clicked Entity
}
}
}