Resolution found!
Using the ToNumber() function on all of the field references in expressions has resolved it:
X&Y = ToNumber(
[Active Screen Bounds.Left])<0 AND ToNumber(
[Active Screen Bounds.Top])<0
etc...------------------------------
Andrzej Silarow
Blue Prism Mentor
Smart Automation Services LTD
Europe/London
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Andrzej Silarow