For some reason Rambox didn’t have an icon on the tasklist and it looked following way:
Adding following lines to the signal handler finally resolved the issue (note that default configuration already has this handlerg):
client.connect_signal("manage", function (c)
if c.class == "Rambox" then
local icon = gears.surface("/home/pmakhov/Pictures/256x256.png")
c.icon = icon._native
icon:finish()
end
end)
Based on this SO answer: stackoverflow.com/a/30379815/1252056