Bug 329184 Detached view that is an activation target candidate can prevent perspective switching
1. Detach the 'Outline' view. 2. Window > Open Perspective > Debug 3. Window > Open Perspective > Java 4. The perspective switch doesn't actually happen. You see the 'Debug' perspective's content with an 'Outline' view detached. Closing the 'Outline' view in the 'Debug' perspective will workaround this problem.
bundles/org.eclipse.e4.ui.workbench/src/org/eclipse/e4/ui/internal/workbench/PartActivationHistory.java
context.activateBranch();
		} else {
			IEclipseContext parent = context.getParent();
			do {
				context.activate();
				context = parent;
				parent = parent.getParent();
			} while (parent.get(MWindow.class) != null);
		}
		queue(part);
	}
	void queue(MPart part) {
		generalActivationHistory.remove(part);
		generalActivationHistory.addFirst(part);
	}
	/**
	 * Checks to see if this element and its parents are actually being rendered.
	 */
	private boolean isValid(MUIElement element) {
		if (element == null || !element.isToBeRendered() || !element.isVisible()) {
			return false;
		}
		if (element instanceof MApplication) {
			return true;
		}
		MUIElement parent = element.getParent();
		if (parent == null && element instanceof MWindow) {
			
			parent = (MUIElement) ((EObject) element).eContainer();
		}