When a path is rendered in the Bookmarks area or as the header line of the
tree, it is truncated if there is no enough space for it. But if a path
contains multi-byte characters, it should be truncated by characters, not
bytes, otherwise the path may be truncated between the bytes of a
multi-byte character. To deal with multi-byte characters, use
strdisplaywidth() instead of len() to get the number of display cells, and
use strcharpart() instead of strpart() to truncate a path.
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