Why do I have a start panel and desktop after removing the desktop manager?
I can't answer this well, but more packages need to be removed. An apt search for 'desktop' showed four hits. desktop-base and desktop-file-utils were removed next:
# apt-get remove desktop-base desktop-file-utils
After rebooting the desktop was still present.
gsettings-desktop-schemas was included in the apt search for 'desktop', but removing it cast too wide a net. Networking gear was going to be removed with it. I did not test its removal because I need networking.
Next was an apt search for 'panel'. I believe lxpanel and lxpanel-data were the important two. After removing lxpanel and lxpanel-data and rebooting the desktop was gone.
# apt-get remove lxpanel lxpanel-data
The Qt program still worked as expected by talking directly to the X server.
Here are the results of searching for "desktop", "start" and "panel". lxpanel and lxpanel-data were the important two. After removing lxpanel and lxpanel-data and rebooting the desktop was gone.
$ apt list --installed | grep -i desktop
desktop-base/stable,now 10.0.2 all [installed]
desktop-file-utils/stable,now 0.23-4 armhf [installed,automatic]
gir1.2-freedesktop/stable,now 1.58.3-2 armhf [installed,automatic]
gsettings-desktop-schemas/stable,now 3.28.1-1 all [installed,automatic]
sound-theme-freedesktop/stable,now 0.8-2 all [installed,automatic]
$ sudo apt-get remove desktop-base desktop-file-utils
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
alsa-base bluealsa fonts-piboto fonts-quicksand gdisk gnome-themes-standard
gstreamer0.10-alsa gstreamer0.10-plugins-base gtk2-engines-clearlookspix
gtk2-engines-pixflat libarchive13 libatasmart4 libavahi-glib1
libblockdev-fs2 libblockdev-loop2 libblockdev-part-err2 libblockdev-part2
libblockdev-swap2 libblockdev-utils2 libblockdev2 libbluetooth3 libgck-1-0
libgcr-base-3-1 libgdata-common libgdata22 libgoa-1.0-0b libgoa-1.0-common
libgphoto2-6 libgphoto2-port12 libgstreamer-plugins-base0.10-0
libgstreamer0.10-0 libimobiledevice6 libjansson4 libldb1 libnfs12 liboauth0
libparted-fs-resize0 libplist3 libplymouth4 libsmbclient libtevent0
libudisks2-0 libusbmuxd4 libxfce4util-bin libxfce4util-common libxfce4util7
libxfconf-0-2 lxplug-bluetooth lxplug-cputemp lxplug-ejecter lxplug-network
lxplug-ptbatt lxplug-volume pi-greeter pimixer pipanel pishutdown
pixflat-icons plymouth plymouth-label plymouth-themes python-talloc
rpd-plym-splash rpd-wallpaper samba-libs udisks2 xfconf
Use 'sudo apt autoremove' to remove them.
The following packages will be REMOVED:
desktop-base desktop-file-utils gvfs gvfs-backends gvfs-common gvfs-daemons
gvfs-fuse gvfs-libs raspberrypi-ui-mods
0 upgraded, 0 newly installed, 9 to remove and 0 not upgraded.
After this operation, 19.3 MB disk space will be freed.
$ apt list --installed | grep -i start
libstartup-notification0/stable,now 0.12-6 armhf [installed,automatic]
$ apt list --installed | grep -i panel
lxpanel-data/testing,now 0.10.0-2+rpt7 all [installed,automatic]
lxpanel/testing,now 0.10.0-2+rpt7 armhf [installed,automatic]