Bug 58777 [Viewers] add API to not deactivate CellEditor on FocusLost
TableViewerImpl installs a FocusListener on CellEditors, which calls applyEditorValue() on focusLost events. This FocusListener must be removed since applying the editor value is not always the wanted behavior on focusLost. Subclasses of CellEditor already handle focus loss in an extensible way: they install their own FocusListener and call a hook method focusLost() (which can be extended or reimplemented by subclasses with special needs). To support content assist in table cells (such as in the Change Method Signature dialog), we need to stay in editing mode even while the proposal list has focus. This is currently not implementable, since TableViewerImpl deactivates the CellEditor even if the CellEditor's focusLost() is reimplemented. The fix is to simply remove the redundant FocusListener in TableViewerImpl.
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/CellEditor.java
public void addListener(ICellEditorListener listener) {
        listeners.add(listener);
    }
    /**
     * Adds a property change listener to this cell editor.
     * Has no effect if an identical property change listener 
     * is already registered.
     *
     * @param listener a property change listener
     */
    public void addPropertyChangeListener(IPropertyChangeListener listener) {
        propertyChangeListeners.add(listener);
    }
    /**
     * Creates the control for this cell editor under the given parent control.
     * <p>
     * This framework method must be implemented by concrete
     * subclasses.
     * </p>
     *
     * @param parent the parent control
     * @return the new control, or <code>null</code> if this cell editor has no control
     */
    protected abstract Control createControl(Composite parent);
    /**
     * Creates the control for this cell editor under the given parent control.
     * 
     * @param parent the parent control
     * @since 2.1
     */