Panes

Split from the pane header, drag the separator to resize, and drag a header onto another pane to swap the two — shells keep running. However narrow a pane gets, the header's Split and Close buttons stay visible; the folder and shell buttons step aside first.
Closing a pane closes the gap; closing the last pane closes the workspace.
The left navbar shows workspaces as a tree (add, rename, reorder; the chevron collapses one): each workspace expands into its panes — click a pane to jump to it, in any workspace; double-click it to give it your own name (it then stays put while the agent keeps retitling; clear the name to go back to the live title, and it survives a restart). A workspace's ⋮ menu has New agent, which opens a fresh pane running the chosen agent in that workspace's folder. Below it, History Session lists the active workspace folder's past agent sessions; drag its top edge to make it taller or shorter (double-click the edge to reset) — the height is remembered. Drag the sidebar's right edge to resize it (double-click the edge to reset); the width is remembered. ⌘/Ctrl + B hides the sidebar.
A pane can switch to a different agent, shell or folder from its header menu — note this restarts the shell.