The default size is to not specify a row height.

When you tick the option to set a row height then you gain the ability to specify a row height (in cm, inches or points) it defaults to 'At Least' so a measurement to go with that of zero would be ignored because each row contains a paragraph that does have some height so this overrides the minimal size.

If you set an exact row height of 0 then the number (rightfully) gets ignored. But if you set it to 0.1cm then that height won't get ignored.

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Andrew Lockton
Chrysalis Design, Melbourne Australia