It’s not working is because yii\web\Response defines its own content property, which is used by response formatters to define the final response body. So $response->content is resolving to that, rather than falling back to the behavior’s content property.

I hadn’t considered that the content may need to be modified after being set. We could possibly rename CpScreenResponseBehavior::content to contentHtml (etc.) in Craft 5 to avoid the name conflict. We’ll discuss that internally.