I decided to encode the entire application state as a Base64 encoded string in the hashmark of the url. For example, a url would look like (note its truncated since they are very long):

Here's the pseudo code for creating the url, and then later reading it:

const stateString = JSON.stringify(appState); // appState is a json object
const compressed = compress(stateString);
const encoded = Base64.encode(compressed);
// Push that `encoded` string to the url
// ... Later, on page load or on undo/redo we read the url and
// do the following
const decoded = Base64.decode(encoded); // same encoded as above, but read from url
const uncompressed = uncompress(decoded);
const newState = JSON.parse(uncompressed);
// Now load your application with the newState

A better idea is to use the URI fragment (the string after the # sign). It has the advantage of not having a size limit, and not needing to be sent all to the backend during the request. window.location.hash is your friend.