Why horizontal tabs? Vertical is navigation. Organizational. Hierarchical. A tree. It's "history". You don't need it on screen all the time. Horizontal is the working set. The threads that are live. Always visible. Give it icons, colors, status. Context at a glance. In apps like ChatGPT, Claude Code or T3 Code, horizontal space is expensive. Tabs at the top let you tuck away the sidebar and leave more room for the right panel: preview, browser, code view. And you still see what's running,…