shadows/framework/src/main/java/org/robolectric/shadows/ShadowDisplayManager.java
public static int addDisplay(String qualifiersStr) {
    int id = getShadowDisplayManagerGlobal().addDisplay(createDisplayInfo(qualifiersStr, null));
    shadowMainLooper().idle();
    return id;
  }
public static int addDisplay(String qualifiersStr) {
    int id = getShadowDisplayManagerGlobal().addDisplay(createDisplayInfo(qualifiersStr, null));
    shadowMainLooper().idle();
    return id;
  }
Adds a simulated display and drain the main looper queue to ensure all the callbacks are
processed.

@param qualifiersStr the {@link Qualifiers} string representing characteristics of the new
display.
@return the new display's ID