mpv + kwin - Hack to make the window translucent when there is no video track

mpv.kwin-translucent-hack.conf

[kwin-translucent-hack]
# Hack to make the window translucent when there is no video track.
# 
# * Set up 2 KWin window rules:
#     * class: exact match `mpv`, title: regular expression `\x034f$`
#       -> active/inactive opacity: force 50%
#     * class: exact match `mpv`, title: regular expression `^[^\x034f]*$`
#       -> active/inactive opacity: force 100%
# 
# * U+034F is used because KWin strips non-printable characters and replaces
#   whitespace sequences with single U+0020 spaces (using `QChar::isPrint()`
#   and `QString::simplify()`, respectively), and U+034F survives those changes.
# 
# * Make sure that there is a U+034F after `${?vid==no:` and
#   the last `${?track-list/count==0:` below
# 
title=${media-title:mpv}${!track-list/count==0:${?vid==no:͏}}${?track-list/count==0:͏}
# Alternative using U+FFFC:
#title=${media-title:mpv}${!track-list/count==0:${?vid==no:}}${?track-list/count==0:}

[example-profile]
force-window=immediate
profile=kwin-translucent-hack