[jdk21u-dev] RFR: 8294148: Support JSplitPane for instructions and test UI

[Andrew Lu]

I backport this for parity with 21.0.5-oracle.

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Commit messages:
 - Backport b419e9517361ed9d28f8ab2f5beacf5adfe3db91

Changes: https://git.openjdk.org/jdk21u-dev/pull/662/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=662&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294148
  Stats: 239 lines in 1 file changed: 214 ins; 4 del; 21 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/662.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/662/head:pull/662

PR: https://git.openjdk.org/jdk21u-dev/pull/662