Bug 323832 [UI] SashContainers are not being removed when unneeded
Start with a fresh workspace Drag the Problems view to the right of its current stack Drag it back... The problem is that the original drag created an MPartSashContainer to host the new stack but the drag back doesn't remove it (i.e. after dragging the Problems view back to its original stack there's now an MPSC with *only* the one stack in it).
bundles/org.eclipse.e4.ui.workbench/src/org/eclipse/e4/ui/workbench/modeling/EModelService.java
public MPlaceholder findPlaceholderFor(MWindow window, MUIElement element);
	/**
	 * Move the element to a new location. The element will be placed at the end of the new parent's
	 * list of children.
	 * 
	 * @param element
	 *            The element to move
	 * @param newParent
	 *            The new parent for the element.
	 */
	public void move(MUIElement element, MElementContainer<MUIElement> newParent);
	/**
	 * Move the element to a new location. The element will be placed at the end of the new parent's
	 * list of children. If 'leavePlaceholder is true then an instance of MPlaceholder will be
	 * inserted into the model at the element's original location.
	 * 
	 * @param element
	 *            The element to move
	 * @param newParent
	 *            The new parent for the element.
	 * @param leavePlaceholder
	 *            true if a placeholder for the element should be added
	 */
	public void move(MUIElement element, MElementContainer<MUIElement> newParent,
			boolean leavePlaceholder);
	/**
	 * Move the element to a new location. The element will be placed at the specified index in the
	 * new parent's list of children.
	 * 
	 * @param element