Bug 319169 [UI] active parts in Detached windows prevent activating a part in the main window
Start with a fresh WS Detach the Outline view using DnD Now you can no longer activate any part in the main window (i.e. Package Explorer). The cause seems to be that the 'rootContainer' has become the DW's MTrimmedWindow rather than the DW's containing MPerspective. This causes the PartService's 'activate' to fail the 'isInContainer()' test...
bundles/org.eclipse.e4.ui.workbench/src/org/eclipse/e4/ui/internal/workbench/ModelServiceImpl.java
/**
	 * Determine if the element passes the matching test for all non-null parameters.
	 * 
	 * @param element
	 *            The element to test
	 * @param id
	 *            The Id
	 * @param clazz
	 *            The class that element must be an instance of
	 * @param tagsToMatch
	 *            The tags to check, <b>all</b> the specified rags must be in the element's tags
	 * @return <code>true</code> iff all the tests pass
	 */
	private boolean match(MUIElement element, String id, Class clazz, List<String> tagsToMatch) {
		if (id != null && !id.equals(element.getElementId()))
			return false;