Hammerspoon script to move/resize window under mouse cursor with modifier keys

init.lua

function get_window_under_mouse()
  -- Invoke `hs.application` because `hs.window.orderedWindows()` doesn't do it
  -- and breaks itself
  local _ = hs.application

  local my_pos = hs.geometry.new(hs.mouse.getAbsolutePosition())
  local my_screen = hs.mouse.getCurrentScreen()

  return hs.fnutils.find(hs.window.orderedWindows(), function(w)
    return my_screen == w:screen() and my_pos:inside(w:frame())
  end)
end

dragging_win = nil
dragging_mode = 1
count = 0

drag_event = hs.eventtap.new({ hs.eventtap.event.types.mouseMoved }, function(e)
  count = count + 1
  if dragging_win then
    local dx = e:getProperty(hs.eventtap.event.properties.mouseEventDeltaX)
    local dy = e:getProperty(hs.eventtap.event.properties.mouseEventDeltaY)
    local mods = hs.eventtap.checkKeyboardModifiers()

    if dragging_mode == 2 and mods.fn and mods.ctrl and (count % 3) == 0 then
      -- Fn + Ctrl to resize the window under cursor
      local sz = dragging_win:size()
      local w1 = sz.w + dx * 2
      local h1 = sz.h + dy * 2
      dragging_win:setSize(w1, h1)
    elseif dragging_mode == 1 and mods.fn then
      -- Fn to move the window under cursor
      dragging_win:move({dx, dy}, nil, false, 0)
    end
  end
  return nil
end)

flags_event = hs.eventtap.new({ hs.eventtap.event.types.flagsChanged }, function(e)
  local flags = e:getFlags()
  drag_event:stop()
  dragging_win = nil
  if flags.fn and flags.ctrl and dragging_win == nil then
    dragging_win = get_window_under_mouse()
    dragging_mode = 2
    count = 0
    drag_event:start()
  elseif flags.fn and not flags.ctrl and dragging_win == nil then
    dragging_win = get_window_under_mouse()
    dragging_mode = 1
    drag_event:start()
  end
  return nil
end)
flags_event:start()