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
return candidate == null ? getActivationCandidate(part) : candidate;
	}
	void forget(MWindow window, MPart part, boolean full) {
		if (full) {
			generalActivationHistory.remove(part);
		} else {
			for (MPlaceholder placeholder : modelService.findElements(window, null,
					MPlaceholder.class, null)) {
				
				if (placeholder.getRef() == part && placeholder.isToBeRendered()) {
					return;
				}
			}
			generalActivationHistory.remove(part);
		}
	}
	MPart getActivationCandidate(MPerspective perspective) {
		for (MPart candidate : generalActivationHistory) {
			if (partService.isInContainer(perspective, candidate)
					&& isValid(perspective, candidate)) {
				return candidate;
			}
		}
		Collection<MPart> candidates = perspective.getContext().get(EPartService.class).getParts();
		for (MPart candidate : candidates) {
			if (isValid(perspective, candidate)) {
				return candidate;
			}
		}
		return null;