backdrop.lua

local wibox = require('wibox')
local gears = require('gears')
local awful = require('awful')

local function update_backdrop(w, c)
  local cairo = require('lgi').cairo
  local geo = c.screen.geometry

  w.x = geo.x
  w.y = geo.y
  w.width = geo.width
  w.height = geo.height

  -- Create an image surface that is as large as the wibox
  local shape = cairo.ImageSurface.create(cairo.Format.A1, geo.width, geo.height)
  local cr = cairo.Context(shape)

  -- Fill with "completely opaque"
  cr.operator = 'SOURCE'
  cr:set_source_rgba(1, 1, 1, 1)
  cr:paint()

  -- Remove the shape of the client
  local c_geo = c:geometry()
  local c_shape = gears.surface(c.shape_bounding)
  cr:set_source_rgba(0, 0, 0, 0)
  cr:mask_surface(c_shape, c_geo.x + c.border_width - geo.x, c_geo.y + c.border_width - geo.y)
  c_shape:finish()

  w.shape_bounding = shape._native
  shape:finish()
  w:draw()
end

local function backdrop(c)
  local function update()
    update_backdrop(c.backdrop, c)
  end
  if not c.backdrop then
    c.backdrop = wibox {ontop = true, bg = '#00000054', type = 'splash'}
    c.backdrop:buttons(
      awful.util.table.join(
        awful.button(
          {},
          1,
          function()
            c:kill()
          end
        )
      )
    )
    c:connect_signal('property::geometry', update)
    c:connect_signal(
      'property::shape_client_bounding',
      function()
        gears.timer.delayed_call(update)
      end
    )
    c:connect_signal(
      'unmanage',
      function()
        c.backdrop.visible = false
      end
    )
    c:connect_signal(
      'property::shape_bounding',
      function()
        gears.timer.delayed_call(update)
      end
    )
  end
  update()
  c.backdrop.visible = true
end

_G.client.connect_signal(
  'manage',
  function(c)
    if c.drawBackdrop == true then
      backdrop(c)
    end
  end
)