Since the upgrade to 9.2.1222, the sidebar stays persistent. Each app launch I seem to have to go to Menu → view → sidebar - hide sidebar.
Also check that you have the Windows OS Display “DPI scaling set to 100%” in Windows Settings.
Lastly if you are using Mutiple monitors with a eg: Non Standard Monitor & unusual / differant resolutions like in this other users Hidden sidebar problem thread that keeps unhiding randomly. That problem appears to be related to multiple monitors if you have those.