ui: create the rolling pane lazily, on its first line

DebUi seeded both widgets with a "(starting...)" placeholder,
replaced as soon as real content arrived during a build. pkh put
reports through the same view but runs no subprocess, so nothing
ever fed the pane: its seed line stayed on screen for the whole
upload, stacked under the per-file byte progress.

Drop the seeds and add the pane bar to the terminal only when the
first classified line arrives; a phase change (and the final
suspend) takes it off again. Flows without subprocess output now
render the status bar alone.

src/ui/deb.rs

@@ -50,8 +50,11 @@ struct Pipeline {
 
 /// State shared between [`DebUi`] and its sinks
 struct Shared {
+    multi: MultiProgress,
     top: ProgressBar,
-    pane: ProgressBar,
+    /// The rolling pane bar, created on demand: flows without subprocess
+    /// output (e.g. `pkh put`) never show it at all
+    pane: Mutex<Option<ProgressBar>>,
     state: Mutex<Pipeline>,
     tee: Mutex<Option<File>>,
     log_path: Mutex<PathBuf>,
@@ -85,34 +88,18 @@ impl DebUi {
             pb.enable_steady_tick(Duration::from_millis(80));
             pb.set_style(spinner_style());
             pb.set_prefix("Building package");
-            pb.set_message("(starting…)");
-            pb
-        } else {
-            ProgressBar::hidden()
-        };
-
-        let pane = if enabled {
-            let pb = multi.add(ProgressBar::new(0));
-            pb.enable_steady_tick(Duration::from_millis(150));
-            // No template margin: multi-line messages are only prefixed by
-            // the template on their first line, which would misalign the
-            // pane; each rendered line carries its own indent instead.
-            pb.set_style(
-                ProgressStyle::default_bar()
-                    .template("{msg}")
-                    .expect("valid template"),
-            );
-            pb.set_message("  │ (starting…)");
             pb
         } else {
             ProgressBar::hidden()
         };
+        let pane = Mutex::new(None);
 
         let timestamp = chrono::Utc::now().format("%Y%m%dT%H%M%S").to_string();
         let log_path = default_log_path(&timestamp);
 
         let ui = Self {
             shared: Arc::new(Shared {
+                multi: multi.clone(),
                 top,
                 pane,
                 state: Mutex::new(Pipeline {
@@ -198,7 +185,7 @@ impl DebUi {
         if self.shared.enabled {
             self.shared.top.set_style(spinner_style());
             self.shared.top.set_message(label.to_string());
-            self.shared.pane.set_message("");
+            drop_pane(&self.shared);
         }
     }
 
@@ -221,9 +208,8 @@ impl DebUi {
             return;
         }
         self.shared.top.disable_steady_tick();
-        self.shared.pane.disable_steady_tick();
+        drop_pane(&self.shared);
         self.shared.top.finish_and_clear();
-        self.shared.pane.finish_and_clear();
     }
 
     /// Success outcome body: clear the widget and print the artifacts,
@@ -408,7 +394,44 @@ fn push_line(shared: &Shared, st: &mut Pipeline, kind: Kind, text: String) {
     let now = Instant::now();
     if now.duration_since(st.last_draw) >= REDRAW_INTERVAL {
         st.last_draw = now;
-        shared.pane.set_message(render_pane(&st.lines));
+        if let Some(pane) = ensure_pane(shared) {
+            pane.set_message(render_pane(&st.lines));
+        }
+    }
+}
+
+/// The pane bar, added to the terminal on the first call and reused after
+///
+/// Returns `None` once the widget is suspended: a line racing the suspend
+/// must not re-add a bar the cleanup just cleared.
+fn ensure_pane(shared: &Shared) -> Option<ProgressBar> {
+    let mut pane = shared.pane.lock().unwrap();
+    if let Some(pb) = pane.as_ref() {
+        return Some(pb.clone());
+    }
+    if shared.suspended.load(Ordering::SeqCst) {
+        return None;
+    }
+    let pb = shared.multi.add(ProgressBar::new(0));
+    pb.enable_steady_tick(Duration::from_millis(150));
+    // No template margin: multi-line messages are only prefixed by the
+    // template on their first line, which would misalign the pane; each
+    // rendered line carries its own indent instead.
+    pb.set_style(
+        ProgressStyle::default_bar()
+            .template("{msg}")
+            .expect("valid template"),
+    );
+    *pane = Some(pb.clone());
+    Some(pb)
+}
+
+/// Take the pane bar off the terminal; the next pushed line re-creates it
+fn drop_pane(shared: &Shared) {
+    if let Some(pb) = shared.pane.lock().unwrap().take() {
+        pb.disable_steady_tick();
+        pb.finish_and_clear();
+        shared.multi.remove(&pb);
     }
 }
 
@@ -638,6 +661,47 @@ mod tests {
         assert!(rendered.contains("a line that would overflow a narrow pane"));
     }
 
+    /// A fresh view has no pane at all: flows without subprocess output
+    /// (`pkh put`) must not render anything until a line arrives, and a
+    /// dropped or suspended pane stays gone
+    #[test]
+    fn pane_is_created_lazily_and_dropped_cleanly() {
+        let multi = MultiProgress::new();
+        let shared = Shared {
+            multi: multi.clone(),
+            top: multi.add(ProgressBar::new(0)),
+            pane: Mutex::new(None),
+            state: Mutex::new(Pipeline {
+                classifier: Box::new(GenericClassifier::new()),
+                lines: VecDeque::new(),
+                errors: Vec::new(),
+                last_draw: Instant::now(),
+                bar_total: 0,
+            }),
+            tee: Mutex::new(None),
+            log_path: Mutex::new(std::env::temp_dir().join("pkh-pane-test.log")),
+            timestamp: String::new(),
+            enabled: true,
+            suspended: AtomicBool::new(false),
+            started: Instant::now(),
+        };
+
+        assert!(shared.pane.lock().unwrap().is_none());
+
+        ensure_pane(&shared).unwrap();
+        assert!(shared.pane.lock().unwrap().is_some());
+        // Later lines hit the stored bar instead of stacking another one
+        ensure_pane(&shared).unwrap();
+        assert!(shared.pane.lock().unwrap().is_some());
+
+        drop_pane(&shared);
+        assert!(shared.pane.lock().unwrap().is_none());
+
+        // A line racing the suspend must not re-add the cleared bar
+        shared.suspended.store(true, Ordering::SeqCst);
+        assert!(ensure_pane(&shared).is_none());
+    }
+
     /// Best-effort ANSI escape stripper, enough for the assertions above
     fn strip_ansi(line: &str) -> String {
         let mut out = String::new();