The macOS Proton Mail app should support the cmd-0 (zero) keyboard shortcut to reopen the main window in case it was closed (without quitting the app). It should also reopen the main window if the user uses cmd-tab to switch to the app and its main window isn't already open.

This is basic, typical behavior for macOS apps. See, e.g., News and Maps, which both support both of these behaviors.

It's super clunky that the only way for me to get Proton Mail to open its main window again is to actually click on it in the dock.