RFR: 8294699: Launcher causes lingering busy cursor

[Alexey Semenyuk]

8294699: Launcher causes lingering busy cursor

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Commit messages:
 - Make sure worker thread terminates before the main thread does.
 - 8294699: Launcher causes lingering busy cursor

Changes: https://git.openjdk.org/jdk/pull/16958/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16958&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294699
  Stats: 78 lines in 1 file changed: 76 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/16958.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16958/head:pull/16958

PR: https://git.openjdk.org/jdk/pull/16958