Hmm can you open the Developer Tools in Mailspring (from the Developer menu), go to the console and type
document.addEventListener('keyup', (e) => console.log(e.keyCode))? Then switch focus back to the Mailspring window and hit the back / forward buttons on the keyboard and let me know what keycodes are in the console.
You can probably map the back/forward “key press” events from your mouse in the keyboard shortcuts panel in Preferences > Keymaps (click a mapping to change it) but I’m curious to know if we can add them by default.
(Originally posted by bengotow on GitHub.)