for real this time – stop appending substrings as tspans if they'd cause the text to overflow the vertical boundaries

The previous commit claiming to this actually didn’t quite get the job
done because I had turned off dev mode and didn’t notice a small
overlap problem. Shifting the y attribute of the entire text node down
by the line height value puts the baseline within the bounds, but that
means it overlaps the bounds because you have one less available
line-height iteration than you might expect. Whoops! This should take
care of it, though.