At one time this was not possible at all in Calibe. Is it possible now and what are the steps one would take to toggle the sidebar?
There are two ways to hide and show the sidebar/topbar in newer versions of Calibe. First, you can click the Settings icon, click Miscellaneous options, and uncheck/check Show controls in the viewer window. This is buried in the options and may be difficult to find.
Now a second, much easier way is available - you can simply right click within the text of the book or document and left click the menu item to show/hide controls.
For macOS 10.14.5 running calibre 3.46.0, I found that CMD+F11 toggles viewing the controls. This does not seem to be documented at https://manual.calibre-ebook.com but you can see it in the context menu:
If you need to maximize the main book view, there are keyboard shortcuts to toggle the Tag Browser (Shift+Alt+T), The Book Detail (Shift+Alt+D) and the Cover Browser (Shift+Alt+B), or you can use the relevant buttons on the bottom right corner of the main window.
If you want to maximize the reading area in the Ebook Viewer integrated in Calibre by hiding the toolbars, you can simply use the F11 key. It will toggle between normal mode and full screen mode.