Bug 294462 [FieldAssist] ControlDecoration#showHoverText doesn't check if the widget is visible
If the HOVER-Text is set on decoration of a widget which is invisible the Hover-Shell is still poping up because it doesn't check if the control is invisible make the Hover appearing somewhere on the screen.
bundles/org.eclipse.jface/src/org/eclipse/jface/fieldassist/ControlDecoration.java
* and the horizontal edge of the control. The default value of this setting
	 * is <code>0</code>.
	 * 
	 * @return the number of pixels that should be reserved between the
	 *         horizontal edge of the control and the adjacent edge of the
	 *         decoration.
	 */
	public int getMarginWidth() {
		return marginWidth;
	}
	/**
	 * Set the margin width in pixels that should be used between the decorator
	 * and the horizontal edge of the control. The default value of this setting
	 * is <code>0</code>.
	 * 
	 * @param marginWidth
	 *            the number of pixels that should be reserved between the
	 *            horizontal edge of the control and the adjacent edge of the
	 *            decoration.
	 */
	public void setMarginWidth(int marginWidth) {
		previousDecorationRectangle = getDecorationRectangle(control.getShell());
		this.marginWidth = marginWidth;
		update();
	}
	/**
	 * Something has changed, requiring redraw. Redraw the decoration and update
	 * the hover text if appropriate.
	 */
	protected void update() {