RFR: 8252236: TabPane: must keep header of selected tab visible [v2]

[Jeanette Winzenburg]

> the issue is that the header of the selected tab is not always visible (or kept visible, see report for details)
> 
> - initially
> - after changing side
> - after resizing stage/tabPane
> - after modifying the list of tabs
> 
> Fixed in TabPaneSkin to notify its TabHeaderArea (== collaborator that is responsible for layout the tabs) whenever its
> scroll might need an update.
> Added tests that fail before and pass after the fix.
> 
> Note that there still are issues while/after dragging, which are not in the scope of this fix.

Jeanette Winzenburg has updated the pull request incrementally with one additional commit since the last revision:

  widened scope of fix to address review comment

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/300/files
  - new: https://git.openjdk.java.net/jfx/pull/300/files/d4c33714..c3583829

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=300&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=300&range=00-01

  Stats: 161 lines in 3 files changed: 159 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/300.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/300/head:pull/300

PR: https://git.openjdk.java.net/jfx/pull/300