This is an example page for Leaflet.ShowAll. Check out the source code, it's super simple!
Leaflet.ShowAll allows you to zoom out to show the entire area, then zoom back in to the same view you were just at. You can move around while in show-all mode, but if you zoom in or out you're out of show-all and your previous view is lost. Here you can also double-click on the marker to go back to the original view.