That is actually part of the "extra support" we decided to pay for.

The application was originally written in Vaadin 7, Vaadin 8 had an absolute ton of breaking changes and we could not migrate in time, and then the breaking changes continued to compound as new releases happened and they migrated away from GWT, making a "big bang" rewrite not practical.

Their extended support included a tool called multi-platform runtime, which allowed the old application and new application to be run side-by-side and each section replaced one at a time. So that is essentially what we did. Still took 3+ years to do the rewrite.