This major release changes the prototype of a jsdom's EventTarget.prototype to point to the Object.prototype inside the jsdom, instead of pointing to the Node.js Object.prototype. Thus, the prototype chain of Window stays entirely within the jsdom, never crossing over into the Node.js realm.
This only occurs when runScripts is set to non-default values of "dangerously" or "outside-only", as with the default value, there is no separate Object.prototype inside the jsdom.
This will likely not impact many programs, but could cause some changes in instanceof behavior, and so out of an abundance of caution, we're releasing it as a new major version.