whitespace.render checked the loop bound only before writing a rune,
then advanced by the rune width. A rune wider than the remaining space
was still written, overshooting the requested width by up to
runeWidth-1 cells, and this made the trailing space-fill unreachable.
Stop before a wide rune would overflow and let the existing space-fill
pad the remaining cells, so the result is always exactly the requested
width. This also fixes overflow surfaced through Place/PlaceHorizontal
with WithWhitespaceChars for multi-cell whitespace characters.
dualfroz
upstream