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
hoverShell.setVisible(true);
				}
			} else {
				if (hoverShell.isVisible()) {
					hoverShell.setVisible(false);
				}
			}
		}
		/*
		 * Set the text of the hover to the specified text. Recompute the size
		 * and location of the hover to hover near the decoration rectangle,
		 * pointing the arrow toward the target control.
		 */
		void setText(String t, Rectangle decorationRectangle,
				Control targetControl) {
			if (t == null) {
				t = EMPTY;
			}
			if (!t.equals(text)) {
				Point oldSize = getExtent();
				text = t;
				hoverShell.redraw();
				Point newSize = getExtent();
				if (!oldSize.equals(newSize)) {
					
					arrowOnLeft = decorationRectangle.x <= targetControl
							.getLocation().x;
					setNewShape();
				}
			}