Thanks for response,
Graphics: Onboard AMD Radeon \ CEZANNE (Not that old.)
'Lock manual resize' is not enabled but I toggled it just to see if that made a difference. It didn't.
If I switch on non-layered mode then the left click context menu appears normally along with the ability to resize. If I switch it back off then both are not possible.
At least I know I can switch on non-layered mode if I need to resize.
Thanks again.