Bug 295386 [FieldAssist] ControlDecoration#showHoverText doesn't check if the widget is visible
bundles/org.eclipse.jface/src/org/eclipse/jface/fieldassist/ControlDecoration.java
* the control has focus. The default value of this setting is
	 * <code>false</code>.
	 * 
	 * @return <code>true</code> if the decoration should only be shown when
	 *         the control has focus, and <code>false</code> if it should
	 *         always be shown. Note that if the control is not capable of
	 *         receiving focus (<code>SWT.NO_FOCUS</code>), then the
	 *         decoration will never show when this value is <code>true</code>.
	 */
	public boolean getShowOnlyOnFocus() {
		return showOnlyOnFocus;
	}
	/**
	 * Set the boolean that controls whether the decoration is shown only when
	 * the control has focus. The default value of this setting is
	 * <code>false</code>.
	 * 
	 * @param showOnlyOnFocus
	 *            <code>true</code> if the decoration should only be shown
	 *            when the control has focus, and <code>false</code> if it
	 *            should always be shown. Note that if the control is not
	 *            capable of receiving focus (<code>SWT.NO_FOCUS</code>),
	 *            then the decoration will never show when this value is
	 *            <code>true</code>.
	 */
	public void setShowOnlyOnFocus(boolean showOnlyOnFocus) {
		this.showOnlyOnFocus = showOnlyOnFocus;
		update();
	}
	/**