RFR: 7093691: Nimbus LAF: disabled JComboBox using renderer has bad font color [v3]

[Damon Nguyen]

> Before the fix, a JComboBox in Nimbus L&F would have normal black text even when the JComboBox was disabled if SynthComboBoxRenderer was replaced with a DefaultListCellRenderer. This text should be greyed out like in other L&F's. When looking into the defaults for Nimbus L&F files for attributes and states of a JComboBox, it confirm that the intention for disabled JComboBoxes is nimbusDisabledText (which is grey text).
> 
> SynthComboBoxes have an additional check in its default SynthComboBoxRenderer that enables/disables the renderer itself. The SynthComboBoxRenderer inherits its enabled state from the parent ComboBox. Since the renderer with DefaultListCellRenderer is in a separate class without a reference to the comboBox, a listener was added to SynthComboBoxUI.
> 
> An additional issue occurred in DefaultListCellRenderer because the renderer overrode the listener's re-assigned enabled state. In testing, setting the enabled state in DefaultListCellRenderer is redundant for all L&F's and is not needed here. However, instead of removing it altogether, a conditional was added specifically to allow ComboBoxes to skip setting enabled state here.
> 
> After the fix, the Nimbus JComboBox with DLCR set matches the appearance of a normal Nimbus JComboBox. I can enable/disable the JComboBoxes in the test, and the UI elements behave and appear as expected.

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

  Simplify Nimbus LAF check

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12390/files
  - new: https://git.openjdk.org/jdk/pull/12390/files/d586b750..fa70b83f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12390&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12390&range=01-02

  Stats: 11 lines in 1 file changed: 0 ins; 8 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12390.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12390/head:pull/12390

PR: https://git.openjdk.org/jdk/pull/12390