sessionStorage. You can use a Singleton to store your form data inside you GWT application, but keep in mind, doing a reload on place 2 will destory your stored data and you need to go back to the first place and the user needs to enter the data again!
Using GWT3 compatible stuff (JsInterop / elemental2).
I want to intercept calls to console.log in my own code. So I can capture all the strings written to the console in my own code.
Can I achieve this in GWT? And how?
sandboxon your iframe (https://developer.mozilla.org/en-US/docs/Web/HTML/Element/iframe#attr-sandbox) where you'd need at least
allow-scripts, possibly more.