spurious characters from previous view in preview window

[ALTree]

$ fzf --version
0.56.0 (ff16877)

Using Bash on Linux. Reproducer:

echo -e "aaaaaaaaaa \nbbbbbbbbb" | fzf --preview-window=10 --preview="echo {}"

First item preview is correct:

The second isn't (note the a at the end):

The last column of the preview window hasn't been cleaned, and it shows the a that was in that position for the previous item.

[junegunn]

Thanks, bacc860 should fix it, but please let me know if you see any regression from the change.