Peter Allan 8212
Do you recall the name of that flag?
The only one I've seen that sounds like something similar to the one you mention is shown below:
Enable per-desk Z-order for all-desk windows.
The Z-order of all-desk windows is maintained on a per-desk basis.
This means that all-desk windows will not keep popping to the front when switching desks.
It was introduced in M110 and will expire in M130.
~DennyL