Problem:
With these two lines (`\b` marking a backspace):
f\bfoo
xb\bb
line 1's bold run ends at byte 1 and line 2's begins at byte 1, so line 1
renders "fo" in bold rather than "f", and line 2's "b" is not bold at all.
Solution:
Only grow a highlight group that is on the current row.
8d9a798d6d