[jdk26] RFR: 8373946: Synth ProgressBarUI implementation confuses background painting with border painting

[Prasanta Sadhukhan]

8373946: Synth ProgressBarUI implementation confuses background painting with border painting

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Commit messages:
 - Backport fe5911c7695283cb76b3d95877138b0b15bc0ba4

Changes: https://git.openjdk.org/jdk/pull/28941/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28941&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373946
  Stats: 237 lines in 3 files changed: 123 ins; 110 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/28941.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28941/head:pull/28941

PR: https://git.openjdk.org/jdk/pull/28941