Glenn,
I developed a fix for this. It has been published several times in this forum, but the search engine is not working, so I cannot find it. I will try to repeat it as much as I can remember.
You need to redefine your TOC paragraph tags. In the paragraph designer, select a TOC tag. Select the Advanced tab. In the section on Word Spacing, set Minimum to 275%, Maximum to 375%, and Optimum to 100%; CLEAR (uncheck) Allow Automatic Letter Spacing. Click Update All.
REPEAT the above for each TOC paragraph tag.
Then update your book.
Normally, Minimum should be less than Optimum, which should be less than Maximum. I have no idea what FrameMaker actually does, but it appears to try to set the word spacing between minimum and maximum; this determines where the line breaks. THEN sets the spacing to optimum, which is 100%. The result is a line break that is several characters to the LEFT of the frame edge. If the above settings do not give you enough break, then add 100% to minimum and maximum, but NOT optimum; optimum should always be 100%. If the breaking is too much, try subtracting 100% from minimum and maximum.
By setting your TOC paragraph tags with these changes, the line breaking is automatic.
Hope this helps,
Van