https://www.reddit.com/r/MacOS/comments/1fmmqj7/severe_focus_loss_bug_still_not_fixed_in_macos/

focus-workaround.lua

-- https://www.hammerspoon.org/

focusWorkaroundWatchers = {}
local lastActivatedApp = nil
local restoreActivationTimer = nil
local lastFocusedWindowBySpace = {}

focusWorkaroundWatchers.application = hs.application.watcher
  .new(function(name, event, app)
    if event == hs.application.watcher.activated then
      if name == "WindowManager" then
        restoreActivationTimer = hs.timer.doAfter(0.1, function()
          if lastActivatedApp ~= nil then
            lastActivatedApp:activate()
          end
        end)
      else
        lastActivatedApp = app
      end
    end
  end)
  :start()

hs.window.filter.default:subscribe(hs.window.filter.windowFocused, function(window, name)
  for _, space in ipairs(hs.spaces.windowSpaces(window)) do
    lastFocusedWindowBySpace[space] = window
  end
end)

focusWorkaroundWatchers.spaces = hs.spaces.watcher
  .new(function()
    lastActivatedApp = nil
    if restoreActivationTimer ~= nil then
      restoreActivationTimer:stop()
      restoreActivationTimer = nil
    end

    if hs.application.frontmostApplication():bundleID() == "com.apple.WindowManager" then
      lastFocusedWindow = lastFocusedWindowBySpace[hs.spaces.focusedSpace()]
      if lastFocusedWindow ~= nil then
        lastFocusedWindow:focus()
      end
    end
  end)
  :start()