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
protected CellEditor(Composite parent) {
        this(parent, defaultStyle);
    }
    /**
     * Creates a new cell editor under the given parent control.
     * The cell editor has no cell validator.
     *
     * @param parent the parent control
     * @param style the style bits
     * @since 2.1
     */
    protected CellEditor(Composite parent, int style) {
        this.style = style;
        create(parent);
    }
    /**
     * Activates this cell editor.
     * <p>
     * The default implementation of this framework method
     * does nothing. Subclasses may reimplement.
     * </p>
     */
    public void activate() {
    }
    /**
     * Adds a listener to this cell editor.
     * Has no effect if an identical listener is already registered.
     *
     * @param listener a cell editor listener
     */