Bug 319169 [UI] active parts in Detached windows prevent activating a part in the main window
Start with a fresh WS Detach the Outline view using DnD Now you can no longer activate any part in the main window (i.e. Package Explorer). The cause seems to be that the 'rootContainer' has become the DW's MTrimmedWindow rather than the DW's containing MPerspective. This causes the PartService's 'activate' to fail the 'isInContainer()' test...
tests/org.eclipse.e4.ui.tests/src/org/eclipse/e4/ui/tests/application/EPartServiceTest.java
public int getActivated() {
			return activated;
		}
		public int getDeactivated() {
			return deactivated;
		}
		public int getHidden() {
			return hidden;
		}
		public int getVisible() {
			return visible;
		}
		public boolean isValid() {
			return valid;
		}
		public List<MPart> getActivatedParts() {
			return activatedParts;
		}
		public List<MPart> getDeactivatedParts() {
			return deactivatedParts;
		}
		public List<MPart> getHiddenParts() {
			return hiddenParts;
		}
		public List<MPart> getVisibleParts() {
			return visibleParts;
		}
		public void partActivated(MPart part) {
			if (valid && part == null) {
				valid = false;