RFR: JDK-8225013: sanity/client/SwingSet/src/ScrollPaneDemoTest.java fails on Linux [v2]

[Alexandre Iline]

> Before the fix the test was failing around 1 time in fifty runs. After the fix the test did not fail in ~ 1k runs.
> Additional instrumentation is added to save useful information on the UI state, should the failure happen again.

Alexandre Iline has updated the pull request incrementally with one additional commit since the last revision:

  Using modern language features.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8848/files
  - new: https://git.openjdk.java.net/jdk/pull/8848/files/03e6c206..2aa1c9e8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8848&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8848&range=00-01

  Stats: 13 lines in 1 file changed: 0 ins; 10 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8848.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8848/head:pull/8848

PR: https://git.openjdk.java.net/jdk/pull/8848