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
if (parent == null) {
			return isValid(partService.getLocalPlaceholder(element));
		}
		return isValid(parent);
	}
	/**
	 * Checks to see if this element and its parents are actually being rendered.
	 */
	boolean isValid(MPerspective perspective, MUIElement element) {
		if (element instanceof MApplication) {
			return true;
		} else if (!element.isToBeRendered() || !element.isVisible()) {
			return false;
		}
		MElementContainer<MUIElement> parent = element.getParent();
		if (parent == null) {
			for (MPlaceholder placeholder : modelService.findElements(perspective, null,
					MPlaceholder.class, null)) {
				if (placeholder.getRef() == element) {
					parent = placeholder.getParent();
					if (!placeholder.isToBeRendered() || !placeholder.isVisible() || parent == null) {
						return false;
					}
					
					if (parent instanceof MGenericStack
							&& parent.getSelectedElement() != placeholder) {
						return false;
					}
					break;
				}