· Interface
Panels, Arc-true navigation, and a flat material
Tabs became panels, the sidebar became the open set with close and delete finally distinct, and the glass material was replaced with a flat frame/panel/raised ladder.
The shell had been built on glass and vibrancy. It was replaced with a flat three-step material (frame, panel, raised) because translucency was carrying decoration rather than depth, and depth is what a pane grid needs.
One item per layout leaf replaced tabs, with a silent migration for existing layouts. The sidebar became the open set: an OPEN group and a SPACE group, with closing a pane and deleting its session distinguished rather than conflated behind one X.
A sidebar item can be dragged onto a panel edge to split there, with the legal drop zones pinned by test rather than by feel.