RFR: 8078744: Right half of system menu icon on title bar does not activate when clicked in Metal L&F

[Prasanta Sadhukhan]

If JFrame's window decorations is provided by the Metal LookAndFeel then system menu icon (e.g., which shows Restore, Minimize, Maximize, and Close menu options when clicked) does not activate when clicked on the right edge of the icon

MetalTitlePane creates a JMenuBar with a system menu icon and a JMenu when clicked on it. This JMenu is created by
`MetalTitlePane.createMenu`  in the title bar with an empty string and zero-width string doesn't cover the systemmenu icon to activate the menu when clicked on right edge of the icon. Changing the text of the JMenu to a non-zero width character properly sizes the menu button covering and activating the system menu icon even when clicked on right edge i.e anywhere in the icon.

Without fix
<img width="246" height="127" alt="image" src="https://github.com/user-attachments/assets/f872bd95-ea15-4245-9c88-f06834804e9b" />

With fix
<img width="190" height="134" alt="image" src="https://github.com/user-attachments/assets/f79e8e96-0c71-49e0-9501-5b3a12bbcd83" />

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Commit messages:
 - 8078744: Right half of system menu icon on title bar does not activate when clicked in Metal L&F

Changes: https://git.openjdk.org/jdk/pull/29808/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29808&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8078744
  Stats: 67 lines in 2 files changed: 65 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/29808.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29808/head:pull/29808

PR: https://git.openjdk.org/jdk/pull/29808