Tooltip: Tooltip might disappear under certain conditions

The issue was observed while code reviewing https://github.com/openjdk/jfx/pull/1394 , with a large graphic set on the tooltip and a very short show delay, but it does happen with the defaults.

To reproduce:
1. run the attached code
2. hover over the label area to show the tooltip
3. try to move the mouse pointer over the tooltip (this may take some trial and error for the issue to show up)
At some point, the tooltip disappears and remains hidden until the mouse pointer is moved again.

It might help to set the show delay to 1 ms:
```
-fx-show-delay: 1ms;
```
(uncomment line 42)

Screen recording:
https://github.com/openjdk/jfx/pull/1394#issuecomment-1986529982