That's a "feature" on Gnome 3.25 but we acknowledge it is inconsistent and imperfect (see also bug 1712992). I'm hearing that either upstream or Ubuntu is going to make a decision/change on the appearance soon.
** Changed in: gnome-shell (Ubuntu) Status: New => Invalid -- You received this bug notification because you are a member of Desktop Packages, which is subscribed to gnome-shell in Ubuntu. https://bugs.launchpad.net/bugs/1715649 Title: gnome panel is only rendered correctly when a window is touching it Status in gnome-shell package in Ubuntu: Invalid Bug description: As described Test case: Log in Notice how light the panel is Open any window, move it up to the panel (if not already Panel is now properly colored ProblemType: Bug DistroRelease: Ubuntu 17.10 Package: gnome-shell 3.25.91-0ubuntu3 ProcVersionSignature: Ubuntu 4.12.0-12.13-generic 4.12.8 Uname: Linux 4.12.0-12-generic x86_64 ApportVersion: 2.20.7-0ubuntu1 Architecture: amd64 CurrentDesktop: ubuntu:GNOME Date: Thu Sep 7 09:57:52 2017 DisplayManager: gdm3 GsettingsChanges: b'org.gnome.shell' b'favorite-apps' b"['firefox.desktop', 'rhythmbox.desktop', 'org.gnome.Nautilus.desktop', 'org.gnome.Terminal.desktop', 'org.gnome.Software.desktop', 'ubuntu-amazon-default.desktop', 'yelp.desktop']" b'org.gnome.desktop.interface' b'gtk-im-module' b"'gtk-im-context-simple'" InstallationDate: Installed on 2017-09-06 (0 days ago) InstallationMedia: Ubuntu 17.10 "Artful Aardvark" - Alpha amd64 (20170904) ProcEnviron: TERM=xterm-256color PATH=(custom, no user) XDG_RUNTIME_DIR=<set> LANG=en_US.UTF-8 SHELL=/bin/bash SourcePackage: gnome-shell UpgradeStatus: No upgrade log present (probably fresh install) To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/gnome-shell/+bug/1715649/+subscriptions -- Mailing list: https://launchpad.net/~desktop-packages Post to : desktop-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~desktop-packages More help : https://help.launchpad.net/ListHelp