I have found a work-around.
Using Sysinternals ProcessMonitor I found that Windows was accessing the following Registry path;
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\GraphicsDrivers\Configuration

My system had three entries;
DELF003YY7707BR0MUL_30_07D7_6A^9A3774EB79DEE3E3E38496CC7DF4D936
QHD32000001_31_07D6_D5^63E1ABDD175E7871DCAEB710418A0F75
SIMULATED_8086_2A42_00000000_00020000_1010100^CDE365D1B3F0942F0CF38BFB8E127AB4

Under each is a tree called "00" two of the keys are
PrimSurfSize.cx
PrimSurfSize.cy

Under "00" was another branch also called "00" two of the keys are;
ActiveSize.cx
ActiveSize.cy

The first two of the configs (ie DELF00... & QHD3...) the above keys were 1440x900,
so they were not involved.

The third (SIMULATED...) were set to 1024x768.

I changed these to 1600x900 and the problem was solved.

Further I changed resolution (via control panel) to 1920x1080, the moving/resize issue returned,
but the lower right corner was set to 1600x900, ie the SIMULATED... settings.

So for some reason one of the configs does not get set correctly.

I don't know why there are three configs, I have only ever used one monitor.

Hope this helps someone out there...