Hammerspoon - maximize main window of app, leave margin all around it

init.lua

-- maximize main window of app, leave margin all around it
appMaximizeWithBorder = function (appBundle)
  local app = hs.application.get(appBundle)
  if app then
    win = app:mainWindow()
    screenSize = win:screen():fullFrame()
    local marginTop = 40
    local marginRBL = 16  -- margin Right, Bottom, Left
    local winRectangle = hs.geometry.rect(
      marginRBL, marginTop,
      screenSize.w-2*marginRBL, screenSize.h-marginTop-marginRBL
    )
    win:setFrame(winRectangle)
  end
end
hs.hotkey.bind({"ctrl"}, "return", function() appMaximizeWithBorder("net.kovidgoyal.kitty") end)