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
private MPart getActivationCandidate(MPart part) {
		
		Collection<MPart> candidates = part.getContext().get(EPartService.class).getParts();
		return findActivationCandidate(candidates, part);
	}
	/**
	 * Finds and returns a part that is a valid candidate to be granted activation.
	 */
	private MPart findActivationCandidate(Collection<MPart> candidates, MPart currentlyActivePart) {
		candidates.remove(currentlyActivePart);
		MPlaceholder activePlaceholder = partService.getLocalPlaceholder(currentlyActivePart);
		for (MPart candidate : candidates) {
			
			if (isValid(candidate)) {
				MPlaceholder placeholder = partService.getLocalPlaceholder(candidate);
				MElementContainer<MUIElement> parent = placeholder == null ? candidate.getParent()
						: placeholder.getParent();
				
				
				if (parent instanceof MGenericStack) {
					MUIElement selection = parent.getSelectedElement();
					
					if (selection == activePlaceholder || selection == currentlyActivePart) {
						return candidate;
					}
					
					if (selection == candidate || selection == placeholder) {
						return candidate;
					}
				} else {