RFR: 8356770: TreeTableView not updated after removing a TreeItem with children and adding it to another parent

Wed Nov 12 15:52:53 UTC 2025

When a subtree is moved in a `TreeTableView` , the visuals don’t update until a resize or expand/collapse. The `TreeTableViewSkin` only rebuilds cells when the expanded row count changes.
This PR makes the skin to detect structural changes on `childrenModificationEvent` using new variable `treeStructureDirty`, and in `updateItemCount()` call `requestRebuildCells()` to refresh the visuals.

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Commit messages:
 - Added new treeStructureDirty

Changes: https://git.openjdk.org/jfx/pull/1971/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1971&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356770
  Stats: 11 lines in 1 file changed: 10 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1971.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1971/head:pull/1971

PR: https://git.openjdk.org/jfx/pull/1971