> It's a pain to use when working with multiple (source independent) projects simultaneously. E.g. a library project and the main project. Eclipse lets you open anything you want to in the same window and presents them in the same explorer view.

That's a common confusion about IntelliJ.

Eclipse Workspace -> IntelliJ Project

Eclipse Project -> IntelliJ Module

In IntelliJ create an empty Project and import all your Maven or Gradle projects as Modules. Everything in one window :)