[jdk11u-dev] RFR: 8015739: Background of JInternalFrame is located out of JInternalFrame

[Goetz Lindenmaier]

I backport this because it fixes an obvious problem. 
The backport is based on the commit to 17.
The test reproduces the issue and passes with the fix.

I had to resolve due to context.
I adapted the test to work with Java 11.

-------------

Commit messages:
 - Backport ...

Changes: https://git.openjdk.org/jdk11u-dev/pull/2607/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2607&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8015739
  Stats: 407 lines in 2 files changed: 360 ins; 8 del; 39 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2607.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2607/head:pull/2607

PR: https://git.openjdk.org/jdk11u-dev/pull/2607