[jdk21u-dev] RFR: 8353007: Open some JComboBox bugs 2

[Satyen Subramaniam]

Backporting JDK-8353007: Open some JComboBox bugs 2. Adds four JComboBox tests. Ran GHA Sanity Checks and new tests directly. Patch is clean. Backporting for parity with Oracle.

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Commit messages:
 - Backport 1889dacb1981d3d15174bc5a201e683a6cdab725

Changes: https://git.openjdk.org/jdk21u-dev/pull/2438/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=2438&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353007
  Stats: 387 lines in 4 files changed: 387 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/2438.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/2438/head:pull/2438

PR: https://git.openjdk.org/jdk21u-dev/pull/2438