Fix: ToolTipHelper doesn't work properly on Linux

The pop-up helper reuses the shell that is used to display the tooltip.
When a new tooltip is shown, the (currently invisible) shell is moved
from its old location and then made visible. This doesn't work in GTK
and the shell remains at its old location.

Instead, the shell is disposed when hiding the pop-up and (if necessary)
a new shell is created when it is shown again.

Resolves #531

org.eclipse.draw2d/src/org/eclipse/draw2d/ToolTipHelper.java

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2000, 2011 IBM Corporation and others.
+ * Copyright (c) 2000, 2024 IBM Corporation and others.
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Public License 2.0 which is available at
@@ -120,6 +120,11 @@ public void displayToolTipNear(IFigure hoverSource, IFigure tip, int eventX, int
 					.getExpanded(getShellTrimSize());
 			setShellBounds(displayPoint.x, displayPoint.y, shellSize.width, shellSize.height);
 			show();
+			// Moving an invisible shell might not be supported by the operating system. Try
+			// again once it has become visible...
+			if (!getShell().getLocation().equals(displayPoint)) {
+				setShellBounds(displayPoint.x, displayPoint.y, shellSize.width, shellSize.height);
+			}
 			currentTipSource = hoverSource;
 			timer = new Timer(true);
 			timer.schedule(new TimerTask() {