z-index issues with cell-title-toolbar

[Yoyokrazy]

• issues with z-index interactions between sash, nb sticky lines, and cell-title-toolbar.
  • the cell-title-toolbar was given the highest z-index to avoid issues with the sash.
  • the sticky lines are given a z-index under the sash to avoid overlapping visual bugs

Need to think about how the cell list view could be reworked to avoid this. The 'overlay' technique creates the z-index issues, so maybe needs a rework to have the cell list view sizing adjust alongside sticky...

[Yoyokrazy]

fixed via #201837