/etc/X11/Xsession.d/01gtk3-nocsd is in gtk3-nocsd 3-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 | # gtk3-nocsd global configuration
#
# This file configures the global behavior of gtk3-nocsd. These
# settings may be overridden by individual users by setting the same
# variables to different values in their ~/.xsessionrc.
#
# Whether to disable CSD on non-GNOME desktops
# 0: disable CSD
# 1: enable CSD
#
# If this setting is left unset, it will default to 0 (CSD disabled)
# unless GTK3_NOCSD_IGNORE is set to 1, in which case it will remain
# unset. (With gtk defaulting to showing CSDs.)
#
#export GTK_CSD=0
#
# Whether to not preload libgtk3-nocsd.so.0 even if GTK_CSD=0
#
# If this setting is set to 1, LD_PRELOAD and GTK_CSD will not be
# touched upon session startup.
#
#export GTK3_NOCSD_IGNORE=0
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