Thanks to Geany-Preview (https://github.com/xiota/geany-preview), one can code within Geany on one side the code in the editor and see in the same window on the other side, in the sidebar, a preview of the code. Geany gives the possibility to switch with a shortcut to the editor and Geany gives the possibility to switch with a shortcut to the sidebar, but with different shortcuts (https://www.geany.org/manual/current/index.html#focus-keybindings). Better would be to use always one shortcut to toggle editor/sidebar.