[jhowell]

I did some more testing of this and it looks like eggheadbooks1 and AaronShep are both right in some cases.

I don't know all of the factors involved, but in my testing as long as there is an average of at least three regular spaces for every one non-breaking space then the nbsp's are retained. If there are too many nbsp's per regular space then they are replaced with regular spaces.

I have attached a sample EPUB that demonstrates nbsp's working in KFX. After opening it in Kindle Previewer 3.8 you can change the font and font size and you will see that it never breaks between 4's and 1's, where I have placed nbsp characters.

I have also attached a screenshot from the Previewer and a partial hex dump of the equivalent KFX file with the utf-8 nbsp characters highlighted.