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 getSiblingActivationCandidate(MPart part) {
		MPlaceholder placeholder = part.getCurSharedRef();
		MUIElement candidate = getSiblingSelectionCandidate(part, placeholder == null ? part
				: placeholder);
		return (MPart) (candidate instanceof MPlaceholder ? ((MPlaceholder) candidate).getRef()
				: candidate);
	}
	private MUIElement getSiblingSelectionCandidate(MPart part, MUIElement element) {
		List<MUIElement> siblings = element.getParent().getChildren();
		for (MPart previouslyActivatedPart : generalActivationHistory) {
			if (previouslyActivatedPart != part && previouslyActivatedPart.isToBeRendered()) {
				if (siblings.contains(previouslyActivatedPart)) {
					return previouslyActivatedPart;
				}
				MPlaceholder placeholder = partService.getLocalPlaceholder(previouslyActivatedPart);
				if (placeholder != null && placeholder.isToBeRendered()
						&& siblings.contains(placeholder)) {
					return placeholder;
				}
			}
		}
		return null;
	}
	MUIElement getSiblingSelectionCandidate(MPart part) {
		MPlaceholder placeholder = part.getCurSharedRef();
		return getSiblingSelectionCandidate(part, placeholder == null ? part : placeholder);
	}
}