I just noticed that the Help button in the Plugin Manager is not working on gtk3. That means, if I click on it nothing happens. Re-compiling Geany and Geany-Plugins for gtk2 made it working again.
The other buttons (Preferences, Keybindings, Close) work without problems.
My environment: Ubuntu 18.04.2, Geany 1.36 (current master), GTK 3.22.30, GLib 2.56.4.