During the review of PR 1506 [1] for JDK-8299738, Andy discovered that the singleton glass Application instance is accessed by multiple threads, without the static `application` field being properly synchronized.

During the normal operation of JavaFX, this isn't a problem, but we need to examine the toolkit startup and shutdown, particularly the latter.

In addition to the Application singleton, there is an `eventThread` variable in that class that should also be examined. Similarly, the Toolkit class should be examined, and particularly the static `fxUserThread` field.

[1] https://github.com/openjdk/jfx/pull/1506#discussion_r1678448367