In my first attempt to organise a fully manual show, I discovered some PTE quirks. Here is one of them.
Thinking slide durations were not wanted, I tried to set them all to zero. However, PTE adjusts the time for each slide to 0.001 seconds.
However, in Project Options it is possible to set Default Slide Duration to zero, and then to Apply to all slides.
If I then try to Preview from a particular side, the program appears to lock up, preventing any further interaction other than closing.