RFR: JDK-8320444: Column drag header is positioned wrong for nested columns [v2]

[Marius Hanl]

> When a nested column is dragged, the column drag header (blue) is positioned wrong.
> It appears at the very left of the table and not where the column is.
> 
> ![image](https://github.com/openjdk/jfx/assets/66004280/6c2a0f61-f32d-4c38-9549-e2e20f5bd4f8)
> 
> The fix is to calculate the bound based off the `TableHeaderRow` (and not the parent `NestedTableColumnHeader`.

Marius Hanl has updated the pull request incrementally with one additional commit since the last revision:

  Improve test code.

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1292/files
  - new: https://git.openjdk.org/jfx/pull/1292/files/6cb62230..976e24ff

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1292&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1292&range=00-01

  Stats: 24 lines in 2 files changed: 10 ins; 0 del; 14 mod
  Patch: https://git.openjdk.org/jfx/pull/1292.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1292/head:pull/1292

PR: https://git.openjdk.org/jfx/pull/1292