fix(desktop): keep tab bar visible when toggling bottom panel panes

When terminal and logs share a zone, toggling one off no longer folds the
entire zone — it switches to the still-open sibling so the tab bar stays
accessible. When the zone does collapse with ≥2 panes, the horizontal tab
bar remains visible (instead of degrading to a vertical rail) so the user
can switch between terminal/logs without expanding first.

- store.ts: add paneOpenGetters registry; setPaneCollapsed checks for an
  open sibling before minimizing the zone
- controller.tsx: bindPaneCollapse registers an open-state getter
- tree-group.tsx: verticalCollapse only for lone panes; ≥2 keeps the strip

apps/desktop/src/app/contrib/controller.tsx

@@ -21,6 +21,7 @@ import {
   registerLayoutResetHandler,
   registerPaneCloser,
   registerPaneOpener,
+  registerPaneOpenGetter,
   resetLayoutTree,
   revealTreePane,
   setPaneCollapsed,
@@ -468,6 +469,7 @@ function bindPaneCollapse(
   $open.listen(isOpen => setPaneCollapsed(paneId, !isOpen))
   registerPaneCloser(paneId, close)
   registerPaneOpener(paneId, open)
+  registerPaneOpenGetter(paneId, () => $open.get())
 }
 
 // SIDES have one source of truth: the TREE. The legacy $panesFlipped flag is

apps/desktop/src/components/pane-shell/tree/renderer/tree-group.tsx

@@ -204,7 +204,13 @@ export function TreeGroup({
   // empty column, so the minimized form is a narrow vertical rail instead
   // (tabs reading top-to-bottom). In a column (stacked zones) the horizontal
   // header IS the collapsed form, exactly as before.
-  const verticalCollapse = Boolean(node.minimized) && parentAxis === 'row' && !isEmpty
+  //
+  // EXCEPTION: when the zone has ≥2 shown panes, keep the horizontal tab bar
+  // even when minimized — the user can still switch between terminal/logs
+  // without expanding the zone first. The vertical rail is only for a lone
+  // pane where the strip adds no value.
+  const verticalCollapse =
+    Boolean(node.minimized) && parentAxis === 'row' && !isEmpty && shown.length <= 1
   const headerVisible = !isEmpty && !verticalCollapse && (Boolean(node.minimized) || !headerHidden)
 
   // Drag handles preventDefault pointerdown (no native dblclick), so the

apps/desktop/src/components/pane-shell/tree/store.ts

@@ -164,6 +164,7 @@ function setDismissed(paneId: string, dismissed: boolean) {
 
 const paneClosers: Record<string, () => void> = {}
 const paneOpeners: Record<string, () => void> = {}
+const paneOpenGetters: Record<string, () => boolean> = {}
 
 /** Route a pane's Close through the app store that owns its visibility. */
 export function registerPaneCloser(paneId: string, close: () => void) {
@@ -181,6 +182,13 @@ export function registerPaneOpener(paneId: string, open: () => void) {
   paneOpeners[paneId] = open
 }
 
+/** Register a getter that reads a tool pane's open/closed state from its
+ *  owning store — so `setPaneCollapsed` can check if a sibling in the same
+ *  zone is still open before folding the whole zone. */
+export function registerPaneOpenGetter(paneId: string, getOpen: () => boolean) {
+  paneOpenGetters[paneId] = getOpen
+}
+
 // TOOL PANELS (terminal, logs, …): their toggle COLLAPSES the zone to a rail
 // (tab stays) instead of hiding it, and the tab's ✕ REMOVES it (vs a session
 // tile, whose ✕ closes the session). Membership tells the renderer which
@@ -1067,7 +1075,18 @@ export function setPaneCollapsed(paneId: string, collapsed: boolean) {
 
         activateTreePane(group.id, group.panes[at - 1] ?? group.panes[at + 1])
       } else {
-        toggleTreeGroupMinimized(group.id, true) // pure tool zone folds as a unit
+        // Pure tool zone: if another tool pane in this zone is still open,
+        // switch to it instead of folding the whole zone — so the tab bar
+        // stays visible and the user can switch between terminal/logs freely.
+        const openSibling = group.panes.find(
+          id => id !== paneId && isCollapsePane(id) && paneOpenGetters[id]?.()
+        )
+
+        if (openSibling) {
+          activateTreePane(group.id, openSibling)
+        } else {
+          toggleTreeGroupMinimized(group.id, true) // no open sibling — fold as a unit
+        }
       }
     } else if (!collapsed) {
       revealTreePane(paneId)