RFR: 8054572: [macosx] JComboBox paints the border incorrectly [v7]

[Damon Nguyen]

> When a JComboBox is editable, the button segment of the combo box is misaligned vertically and has a different height. This change fixes these issues and adds a manual test that checks the appearance of an editable and non-editable JComboBox.
> 
> One of the discussions revolving this issue is the native macOS appearance of editable JComboBoxes. After looking through native macOS apps, the only one found is in System Preferences > Date & Time. The problem here is that the native equivalent found here uses a blue button with a single down arrow as the button's symbol. The current swing implementation uses a white button with an up & down arrow symbol for the button. A JRS widget button that has this blue button with a single downward arrow exists but does not support text fields.
> 
> As such, I believe the best fix for this issue is to mainly fix the alignment and sizing issue. I looked through Apple's documentation for these UI elements but editable JComboBoxes aren't specifically listed anywhere. Similarly, there's barely any editable JComboBoxes used in native mac apps (only the date & time). So, I don't think it's a major issue if JComboBox does not exactly match the example found in Date & Time.

Damon Nguyen has updated the pull request incrementally with one additional commit since the last revision:

  Updated copyright date. Added MacOS text to test.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9473/files
  - new: https://git.openjdk.org/jdk/pull/9473/files/fe0d2f81..0eae1016

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9473&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9473&range=05-06

  Stats: 4 lines in 2 files changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/9473.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9473/head:pull/9473

PR: https://git.openjdk.org/jdk/pull/9473