One minute was not enough - I moved the xwidget startup from being
configured within the xwidget engine's preferences to the Windows
scheduler. Xwidget now starts some 15 minutes after the system is
restarted. It does improve the problem of the white backgrounds showing
on top of the widget's main window, however the problem is not gone
completely. When the system is being heavily used and an xwidget is
also in operation a white box will draw around the widget until the contention is resolved.
The above image shows the problem occurring around a series of Yahoo widgets, the issue affects both engines in the same way so the picture is relevant.
Moving the xwidget startup has a pleasant side-effect - the system
starts much quicker too. There is obviously some contention on startup
as you'd expect and having xwidget in there competing for resources
slows things right down.