[jdk13u-dev] RFR: 7185258: [macosx] Deadlock in SunToolKit.realSync()

[Olga Mikhaltsova]

I'd like to backport JDK-7185258 to jdk13u for parity with jdk11u.
The original patch applied cleanly.
Tested manually with test/jdk/java/awt/dnd/DragWaitForIdle/DragWaitForIdle.java added in this patch.
No regression in tests.

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Commit messages:
 - Backport 14b7dd40905f649f68bbe0c1e0faadd1dfabea84

Changes: https://git.openjdk.java.net/jdk13u-dev/pull/144/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk13u-dev&pr=144&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-7185258
  Stats: 137 lines in 6 files changed: 125 ins; 2 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk13u-dev/pull/144.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk13u-dev pull/144/head:pull/144

PR: https://git.openjdk.java.net/jdk13u-dev/pull/144