Used setOpaque() method to set correct background of platform window Extracted setting background to the particular method in PlatformWindow. Provided appropriate implementation in CPlatformWindow.
------------- Commit messages: - 8273355: Flickering on tooltip appearance IntelliJ IDEA 2021.2.1 Changes: https://git.openjdk.java.net/jdk/pull/7931/files Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7931&range=00 Issue: https://bugs.openjdk.java.net/browse/JDK-8273355 Stats: 25 lines in 5 files changed: 25 ins; 0 del; 0 mod Patch: https://git.openjdk.java.net/jdk/pull/7931.diff Fetch: git fetch https://git.openjdk.java.net/jdk pull/7931/head:pull/7931 PR: https://git.openjdk.java.net/jdk/pull/7931
