Hammerspoon script to mimic MS Windows behavior when clicking on un-focused windows.

clickthrough.lua

-- Description:
--   Mimics Microsoft Windows click-through
--   behavior when changing focused window.
-- Installation:
--   Add anywhere to your Hammerspoon config.
-- Hammerspoon modules used:
--   hs.eventtap, hs.eventtap.event, hs.mouse, hs.timer, hs.window.filter
-- Compatibility:
--   Tested with Hammerspoon 0.9.90 and macOS 10.15.4 (Catalina)
-- Todo:
--   1. Right- and middle-click? 2. Application opt-out filter.
-- License:
--   Public Domain

local evt <const> = hs.eventtap.event

-- Sequence overview:
--   1a. MOUSE BUTTON PRESSED DOWN (or trackpad drag started)
--     2. mouse down event
--       3. window focus changed
--         4. post virtual mouse down event (enables step 1b)
--           5. script is unaffected by it, but target app receives the event
--             6. virtual mouse up event occurs (HS bug) and we suppress it
--   1b. MOUSE CAN BE DRAGGED AROUND to select text or whatnot
--   1c. MOUSE BUTTON RELEASED (or trackpad drag ended)
--     2. mouse up event
-- 
--   1. TRACKPAD TAPPED (briefly)
--     2. mouse down event
--       3. window focus changed
--         4a. mouse up event (while we're still responding to step 3)
--         4b. post virtual mouse down event
--           5. script sees it and posts a virtual mouse up event
--             6. script is unaffected, but target app receives both events (in the correct order hopefully)

local clickthroughInProgress
local virtualEventNumber -- need to keep track of it in order to work around HS bug

local function isVirtual (event)
  return virtualEventNumber == event:getProperty(evt.properties.mouseEventNumber)
end

local function mouseDown (event)
  -- check if virtual down-event occured AFTER corresponding up-event (HS bug)
  if isVirtual(event) and not clickthroughInProgress then
    -- post another up-event to get it unstuck
    local cursorPos = hs.mouse.absolutePosition()
    local mouseUpEvent = evt.newMouseEvent(evt.types.leftMouseUp, cursorPos)
    mouseUpEvent:setProperty(evt.properties.mouseEventNumber, 1)
    mouseUpEvent:post()
    return false -- race condition here? (we want the up-event to occur last), but it always works for me
  end
  clickthroughInProgress = true
  
  -- dirty fix for erratic behavior when dragging items between apps
  -- makes windowFocusChanged inert after 0.1 seconds
  hs.timer.doAfter(0.1, function() clickthroughInProgress = false end)
  
  return false
end

local function mouseUp (event)
  clickthroughInProgress = false
  return isVirtual(event) -- ignore extraneous up-event (HS bug)
end

local function windowFocusChanged ()
  if clickthroughInProgress then
    -- post another down-event so we can continue dragging (to select text or whatnot)
    local cursorPos = hs.mouse.absolutePosition()
    local mouseDownEvent = evt.newMouseEvent(evt.types.leftMouseDown, cursorPos)
    virtualEventNumber = mouseDownEvent:getProperty(evt.properties.mouseEventNumber)
    mouseDownEvent:post() -- causes extraneous up-event when dragging & switching window (HS bug)
  end
end

listener1 = hs.eventtap.new({evt.types.leftMouseDown}, mouseDown):start()
listener2 = hs.eventtap.new({evt.types.leftMouseUp}, mouseUp):start()

-- unfortunately hs.window.filter may cause a bit of unresponsiveness during config load
hs.window.filter.default:subscribe(hs.window.filter.windowFocused, windowFocusChanged)

Known issues

• Not working: Finder, iTerm2, Disk Utility, and probably others
• [FIXED] click gets stuck when using trackpad (Hammerspoon bug)
• [FIXED] stops working after a few minutes (garbage collector releases listener1 and listener2)