fix: keep sidebar collapse clickable with agent scrollbars (#4734)

* fix: keep sidebar collapse clickable with agent scrollbars

refs #4692

* docs: document sidebar mouse handling and regression coverage

refs #4692

src/client/shell/mouse.rs

@@ -676,6 +676,10 @@ impl ClientShellState {
         }
     }
 
+    /// Routes mouse input through overlays, shell controls, and pane interactions.
+    ///
+    /// Hit-test order determines which overlapping control receives the event;
+    /// the sidebar toggle takes precedence over the agent scrollbar beneath it.
     pub(super) fn handle_mouse(&mut self, mouse: MouseEvent, outcome: &mut ClientShellInput) {
         self.update_link_hover(mouse, outcome);
         let point = (mouse.column, mouse.row);
@@ -1935,9 +1939,17 @@ impl ClientShellState {
                 self.workspace_press = None;
                 self.tab_press = None;
                 self.chrome_drag = None;
-                if super::contains(self.hits.sidebar_divider, point)
-                    && !super::contains(self.hits.sidebar_toggle, point)
-                {
+                // The toggle is painted over the agent scrollbar's last cell.
+                if super::contains(self.hits.sidebar_toggle, point) {
+                    self.sidebar_collapsed = !self.sidebar_collapsed;
+                    self.sidebar_collapsed_manual = true;
+                    self.invalidate_pane_surface();
+                    outcome.repaint = true;
+                    outcome.resize = true;
+                    self.persist_chrome_preferences(outcome);
+                    return;
+                }
+                if super::contains(self.hits.sidebar_divider, point) {
                     let now = std::time::Instant::now();
                     let double_click = self.last_sidebar_divider_click.is_some_and(|last| {
                         now.duration_since(last) <= std::time::Duration::from_millis(350)
@@ -2075,15 +2087,6 @@ impl ClientShellState {
                     outcome.repaint = true;
                     return;
                 }
-                if super::contains(self.hits.sidebar_toggle, point) {
-                    self.sidebar_collapsed = !self.sidebar_collapsed;
-                    self.sidebar_collapsed_manual = true;
-                    self.invalidate_pane_surface();
-                    outcome.repaint = true;
-                    outcome.resize = true;
-                    self.persist_chrome_preferences(outcome);
-                    return;
-                }
                 let group_toggle = self.hits.workspaces.iter().find_map(|hit| {
                     let (rect, key) = hit.group_toggle.as_ref()?;
                     super::contains(*rect, point).then(|| (hit.endpoint_id.clone(), key.clone()))

src/client/shell/tests/endpoints.rs

@@ -247,6 +247,54 @@ fn state_with_scrollable_agents() -> (ClientShellState, ClientEndpointId) {
     (state, remote)
 }
 
+/// Checks that the visible toggle wins overlapping scrollbar clicks and can reopen
+/// the sidebar, for both endpoint layouts and both ends of the overflowing list.
+#[test]
+fn sidebar_toggle_remains_clickable_with_overflowing_agents() {
+    for saved_machine in [false, true] {
+        for scroll_to_bottom in [false, true] {
+            let (mut state, _) = state_with_scrollable_agents();
+            if !saved_machine {
+                state.set_endpoint_catalog(&[]);
+            }
+            state.agent_scroll = if scroll_to_bottom { usize::MAX } else { 0 };
+            let frame = state.compose(100, 28).expect("overflowing agent panel");
+            assert!(!state.hits.agent_scrollbar.is_empty());
+            let scroll = state.agent_scroll;
+
+            let toggle = state.hits.sidebar_toggle;
+            let buffer = frame.to_ratatui_buffer().expect("sidebar buffer");
+            assert_eq!(buffer[(toggle.x, toggle.y)].symbol(), "«");
+            let outcome = state.handle_raw_events(vec![RawInputEvent::Mouse(MouseEvent {
+                kind: MouseEventKind::Down(MouseButton::Left),
+                column: toggle.x,
+                row: toggle.y,
+                modifiers: KeyModifiers::NONE,
+            })]);
+            assert!(
+                state.sidebar_collapsed,
+                "collapse with saved_machine={saved_machine}, scroll_to_bottom={scroll_to_bottom}"
+            );
+            assert!(state.sidebar_collapsed_manual);
+            assert!(outcome.repaint && outcome.resize);
+            assert_eq!(state.agent_scroll, scroll);
+            assert!(state.chrome_drag.is_none());
+
+            state.compose(100, 28).expect("collapsed sidebar");
+            let toggle = state.hits.sidebar_toggle;
+            let outcome = state.handle_raw_events(vec![RawInputEvent::Mouse(MouseEvent {
+                kind: MouseEventKind::Down(MouseButton::Left),
+                column: toggle.x,
+                row: toggle.y,
+                modifiers: KeyModifiers::NONE,
+            })]);
+            assert!(!state.sidebar_collapsed);
+            assert!(outcome.repaint && outcome.resize);
+            assert!(state.chrome_drag.is_none());
+        }
+    }
+}
+
 #[test]
 fn agent_navigation_reveals_offscreen_targets() {
     use crate::input::KeybindAction;