Many thanks for the ideas. The machine gives a rating of about 2300 on geekbench.

I disabled the RT speedup, and it seemed to fix the problem. Except that when I re-enabled the speedup, it still worked properly, so I need to discover just what it is that triggers the problem exactly.

As it's a virtual machine, the driver for the 'virtual card' is supplied by VMWare. That may get updated from time to time. However, I did try changing it's mode to 16 bit rather than high colour. This would lighten the load on the driver somewhat.

I also found a setting in BIAB's 'display' settings to 'auto-adjust # of rows' which was unchecked. Enabling it seems to help it make more sensible font size choices, and to fit the chords and rows better, especially when resizing.

So, things are much better, for now, touch wood .