Perhaps by coloring the pane’s title bar (the one that has breadcrumbs, name, search, 3-dot menu, and close X)
This feature request now says “Implemented”. What is (was) the actual implementation?
The only way I can tell now which pane is active is by manually hovering over each of the panes individually, and then looking for the 3 dot menu that appears immediately to the left of the hovered bullet.
This is not the same as having a static visual indicator.
