Thanks for that. You all get the same delay as I do with WM_WINDOWPOSCHANGED. It only gets to LFS when you restore the Window, after that time gap. So that message is no help at all.
The problem, I believe, is that in Windows 10 the WM_ACTIVATE message does not have the "minimised" value set.
WM_ACTIVATE - WA_INACTIVE / minimised = 0
Maybe I can just get around this by taking the other part of the WM_ACTIVATE message - WA_INACTIVE. Because if the window is full screen then it should be 'active' which means it receives keyboard input. Maybe it is enough to assume that if LFS is in full screen mode, but gets a WA_INACTIVE in a WM_ACTIVATE message, then it can assume it is minimised.
By the way, RC-Maus - I think you posted my log by mistake.