RFR: 8283509: Invisible menus can lead to IndexOutOfBoundsException

[Robert Lichtenberger]

findSibling adapted to only use visible menus when calculating the
index.

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Commit messages:
 - 8283509: Invisible menus can lead to IndexOutOfBoundsException

Changes: https://git.openjdk.java.net/jfx/pull/759/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=759&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283509
  Stats: 33 lines in 2 files changed: 22 ins; 4 del; 7 mod
  Patch: https://git.openjdk.java.net/jfx/pull/759.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/759/head:pull/759

PR: https://git.openjdk.java.net/jfx/pull/759