> there's a gain in both wealth and employment if some of those chaps get organized and start fixing windows.
This is the core of the fallacy. There'd be more net wealth without breaking the window in the first place, leaving the guy free to spend that money on someone else's goods/services, who would cause the exact same amount of growth but you'd still have a window.
Spending that money that would have been spent replacing broken windows elsewhere may even generate jobs for the guys who are slack.
You can never stimulate a growth of wealth by destroying wealth. If you're saying the window is already broken, that's not the broken window fallacy.