Avoid row breaking at inconvenient places.

Avoid row breaking at inconvenient places.

The test that was used to avoid breaking a string that was followed
by a too long element was not correct (especially the part that
compared with total row width).

Typical example here is:
- a word with a part that has a font change like /un/breakable;
- a longish sentence after it.

Use a new test that is good enough for this particular case, although
with sortcomings. I do not want to overcomplicate and prefer to wait
for other complaints (this code is already more complicated that I
would like).

Document known shortcoming.

Fix ticket #12660.