Prerequisite
Make sure that one of the options Open project in a new window or Confirm window to open project in in the General page of the Settings dialog is selected.
Opening project in a new window
- Open a project, while another one is already open.
- Depending on the selected behavior:
- If the option Open project in a new window is selected, IntelliJ IDEA will silently open new project in a new window.
- If the option Confirm window to open project in, IntelliJ IDEA
will prompt you to specify whether you want to open project in a new window, or close the current project and reuse the existing window.

Click the New Window button to leave the current project in its window and open the new one in a separate window.
Click This Window button to close the current project and opening the new one in the same window.

