Giving very long lines to libass freezes mpv and makes the CPU spin
(though it's fine with terminal output). This is often the case
user-data/mpv/ytdl/json-subprocess-result in the property list. So limit
the length of selectable items to 300 characters (not Unicode aware).
This is enough to fill a 1920x1080 window with font_size=16 and Japanese
text.