I thought I had mentioned it already or maybe I did not edit my first message.
It behaves exactly the same with default pocillo theme / icons / widgets. Here on another user with default settings :
( by the way that picture also illustrates the problem about overlapping panel / height mismatch, see Top horizontal panel overlapping, if height < 41 px ).
Am I missing some package⋅s in the gtk realm ?
coeur-noir@asgard:~$ dpkg -l | grep gtk
ii adapta-gtk-theme 3.95.0.11-0ubuntu1~bionic1 all Adapta Gtk+ Theme
ii adobe-flash-properties-gtk 1:20191009.1-0ubuntu0.18.04.1 amd64 GTK+ control panel for Adobe Flash Player plugin
ii appmenu-gtk-module-common 0.6.94-1 all Common files for GtkMenuShell D-Bus exporter
ii appmenu-gtk2-module:amd64 0.6.94-1 amd64 GtkMenuShell D-Bus exporter (GTK+2.0)
ii appmenu-gtk3-module:amd64 0.6.94-1 amd64 GtkMenuShell D-Bus exporter (GTK+3.0)
ii apport-gtk 2.20.9-0ubuntu7.9 all GTK+ frontend for the apport crash report system
ii gir1.2-gtk-3.0:amd64 3.22.30-1ubuntu4 amd64 GTK+ graphical user interface library -- gir bindings
ii gir1.2-gtkchamplain-0.12:amd64 0.12.16-2 amd64 Gtk+ widget to display maps (GObject introspection)
ii gir1.2-gtkclutter-1.0:amd64 1.8.4-3 amd64 GObject introspection data for the GTK+ Clutter library
ii gir1.2-gtksource-3.0:amd64 3.24.7-1 amd64 gir files for the GTK+ syntax highlighting widget
ii gir1.2-javascriptcoregtk-4.0:amd64 2.26.2-0ubuntu0.18.04.1 amd64 JavaScript engine library from WebKitGTK+ - GObject introspection data
ii gstreamer1.0-gtk3:amd64 1.14.5-0ubuntu1~18.04.1 amd64 GStreamer plugin for GTK+3
ii gtk-update-icon-cache 3.22.30-1ubuntu4 amd64 icon theme caching utility
ii gtk2-engines-murrine:amd64 0.98.2-2ubuntu1 amd64 cairo-based gtk+-2.0 theme engine
ii gtk2-engines-pixbuf:amd64 2.24.32-1ubuntu1 amd64 pixbuf-based theme for GTK+ 2.x
ii gtk3-nocsd 3-1ubuntu1 all Disable Gtk+ 3 client side decorations (CSD)
ii gtkdialog 2:0.8.3-2 amd64 GUI-creation command-line utility based on GTK+ library
ii handbrake-gtk 1:1.2.1-zhb-1ppa1~bionic1 amd64 versatile DVD ripper and video transcoder - GTK GUI
ii ibus-gtk:amd64 1.5.17-3ubuntu5.2 amd64 Intelligent Input Bus - GTK+2 support
ii ibus-gtk3:amd64 1.5.17-3ubuntu5.2 amd64 Intelligent Input Bus - GTK+3 support
ii libappmenu-gtk2-parser0:amd64 0.6.94-1 amd64 GtkMenuShell to GMenuModel parser (GTK+2.0)
ii libappmenu-gtk3-parser0:amd64 0.6.94-1 amd64 GtkMenuShell to GMenuModel parser (GTK+3.0)
ii libavahi-ui-gtk3-0:amd64 0.7-3.1ubuntu1.2 amd64 Avahi GTK+ User interface library for GTK3
ii libcanberra-gtk-module:amd64 0.30-5ubuntu1 amd64 translates GTK+ widgets signals to event sounds
ii libcanberra-gtk0:amd64 0.30-5ubuntu1 amd64 GTK+ helper for playing widget event sounds with libcanberra
ii libcanberra-gtk3-0:amd64 0.30-5ubuntu1 amd64 GTK+ 3.0 helper for playing widget event sounds with libcanberra
ii libcanberra-gtk3-module:amd64 0.30-5ubuntu1 amd64 translates GTK3 widgets signals to event sounds
ii libchamplain-gtk-0.12-0:amd64 0.12.16-2 amd64 Gtk+ widget to display maps
ii libcheese-gtk25:amd64 3.28.0-1ubuntu1 amd64 tool to take pictures and videos from your webcam - widgets
ii libclutter-gtk-1.0-0:amd64 1.8.4-3 amd64 Open GL based interactive canvas library GTK+ widget
ii libcolord-gtk1:amd64 0.1.26-2 amd64 GTK+ convenience library for interacting with colord
ii libdbusmenu-gtk3-4:amd64 16.04.1+18.04.20171206-0ubuntu2 amd64 library for passing menus over DBus - GTK+ version
ii libdbusmenu-gtk4:amd64 16.04.1+18.04.20171206-0ubuntu2 amd64 library for passing menus over DBus - GTK+ version
ii libgtk-3-0:amd64 3.22.30-1ubuntu4 amd64 GTK+ graphical user interface library
ii libgtk-3-bin 3.22.30-1ubuntu4 amd64 programs for the GTK+ graphical user interface library
ii libgtk-3-common 3.22.30-1ubuntu4 all common files for the GTK+ graphical user interface library
ii libgtk2-perl 2:1.24992-1build1 amd64 Perl interface to the 2.x series of the Gimp Toolkit library
ii libgtk2.0-0:amd64 2.24.32-1ubuntu1 amd64 GTK+ graphical user interface library
ii libgtk2.0-bin 2.24.32-1ubuntu1 amd64 programs for the GTK+ graphical user interface library
ii libgtk2.0-common 2.24.32-1ubuntu1 all common files for the GTK+ graphical user interface library
ii libgtk3-nocsd0:amd64 3-1ubuntu1 amd64 Library to disable Gtk+ 3 client side decorations (CSD)
ii libgtk3-perl 0.032-1 all Perl bindings for the GTK+ graphical user interface library
ii libgtkd-3-0 3.7.5-2build1 amd64 GTK+ graphical user interface library - D bindings
ii libgtkglext1:amd64 1.2.0-8 amd64 OpenGL Extension to GTK+ (shared libraries)
ii libgtkmm-2.4-1v5:amd64 1:2.24.5-2 amd64 C++ wrappers for GTK+ (shared libraries)
ii libgtkmm-3.0-1v5:amd64 3.22.2-2 amd64 C++ wrappers for GTK+ (shared libraries)
ii libgtksourceview-3.0-1:amd64 3.24.7-1 amd64 shared libraries for the GTK+ syntax highlighting widget
ii libgtksourceview-3.0-common 3.24.7-1 all common files for the GTK+ syntax highlighting widget
ii libgtkspell0:amd64 2.0.16-1.2 amd64 spell-checking addon for GTK's TextView widget
ii libido3-0.1-0:amd64 13.10.0+17.04.20161028-0ubuntu1 amd64 Shared library providing extra gtk menu items for display in
ii libjavascriptcoregtk-4.0-18:amd64 2.26.2-0ubuntu0.18.04.1 amd64 JavaScript engine library from WebKitGTK+
ii libnm-gtk0:amd64 1.8.10-2ubuntu3 amd64 library for wireless and mobile dialogs (libnm-glib version)
ii libreoffice-gtk3 1:6.0.7-0ubuntu0.18.04.10 amd64 office productivity suite -- GTK+ 3 integration
ii libwebkit2gtk-4.0-37:amd64 2.26.2-0ubuntu0.18.04.1 amd64 Web content engine library for GTK+
ii libwebkit2gtk-4.0-37-gtk2 2.26.2-0ubuntu0.18.04.1 all Transitional dummy package
ii libwmf0.2-7-gtk 0.2.8.4-12 amd64 Windows metafile conversion library
ii libwxgtk3.0-0v5:amd64 3.0.4+dfsg-3 amd64 wxWidgets Cross-platform C++ GUI toolkit (GTK+ runtime)
ii libwxgtk3.0-gtk3-0v5:amd64 3.0.4+dfsg-3 amd64 wxWidgets Cross-platform C++ GUI toolkit (GTK+ 3 runtime)
ii materia-gtk-theme 20180321-1 all Material Design theme for GNOME/GTK+ based desktop environments
ii overlay-scrollbar-gtk2:amd64 0.2.17.1+16.04.20151117-0ubuntu2 amd64 GTK 2 module for overlay scrollbars
ii paper-gtk-theme 2.1+r265~daily~ubuntu16.10.1 all Paper GTK Theme
ii python-gtk2 2.24.0-5.1ubuntu2 amd64 Python bindings for the GTK+ widget set
ii python-wxgtk3.0 3.0.2.0+dfsg-7 amd64 Python interface to the wxWidgets Cross-platform C++ GUI toolkit
ii python3-aptdaemon.gtk3widgets 1.1.1+bzr982-0ubuntu19.1 all Python 3 GTK+ 3 widgets to run an aptdaemon client
ii qt5-gtk-platformtheme:amd64 5.9.5+dfsg-0ubuntu2.4 amd64 Qt 5 GTK+ 3 platform theme
ii software-properties-gtk 0.96.24.32.11 all manage the repositories that you install software from (gtk)
ii transmission-gtk 2.92-3ubuntu2 amd64 lightweight BitTorrent client (GTK+ interface)
ii ubuntu-release-upgrader-gtk 1:18.04.36 all manage release upgrades
ii usb-creator-gtk 0.3.5ubuntu18.04.2 amd64 create a startup disk using a CD or disc image (for GNOME)
ii xdg-desktop-portal-gtk 1.2.0-flatpak1~bionic amd64 GTK+/GNOME portal backend for xdg-desktop-portal
ii xdg-user-dirs-gtk 0.10-2 amd64 tool to manage well known user directories (Gtk extension)
coeur-noir@asgard:~$
Or do I have too much here ?