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, what's done, what needs your input.
@bcherny@BenjaminDEKR Happens in web app for me quite often as well. Was bugged in a project where calling on context would break the context limit even when all memories, prj files wiped.