Welcome to JustAnswer!
I am Pete, and I'll be happy to answer your question today.
You will need to turn on Word Wrap. Right-click the textbox and choose Format Textbox. Click the Text Box tab and then turn on word wrap.
Word wrap is already on. This problem dpoes not occur if I use a semi-colon ";" instead of a comma "," so it is clearly specific to the comma, and not a generic issue of line length or word-wrapping. Thus if a comma that has caused an unwanted line break is replaced by a semi-colon, the word jumps back to the previous line
I am not sure you can reach me via email
Fairly desperate for a quick solution so I can get to bed with a finished slide presentation for tomorrow ... !
I hope that helped. Keep me posted with your results so that I can help you further.