fix(dock): single-panel drag preview is one full-height slot

drop_index previously mapped a pointer position inside a lone full-height panel
to a top/bottom insertion split, offering an impossible drop. Clamp to the
single slot when the edge holds one panel, and cover the multi-slot mapping with
tests.

src/ui/dock.rs

@@ -211,6 +211,11 @@ pub fn drop_index(y: f32, total: usize, avail: f32) -> usize {
     }
     let h = avail / total as f32;
     let idx = (y / h).round() as usize;
+    // A single full-height panel must not offer a top/bottom split: any drop
+    // lands in the one available slot.
+    if total <= 1 {
+        return 0;
+    }
     idx.min(total)
 }
 
@@ -297,6 +302,24 @@ mod tests {
         assert_eq!(drop_index(50.0, 0, 300.0), 0);
     }
 
+    #[test]
+    fn drop_index_single_slot_is_always_zero() {
+        // One panel sharing the full edge height: any pointer y lands in the
+        // single slot, so the insertion index must be 0 (no top/bottom split).
+        assert_eq!(drop_index(5.0, 1, 900.0), 0);
+        assert_eq!(drop_index(450.0, 1, 900.0), 0);
+        assert_eq!(drop_index(895.0, 1, 900.0), 0);
+    }
+
+    #[test]
+    fn drop_index_multi_slot_maps_position() {
+        // Two panels on an edge produce insertion positions 0..=total: pointer
+        // near the top lands before the first slot (0); pointer near the bottom
+        // of the last slot lands after the last (== total, append).
+        assert_eq!(drop_index(10.0, 2, 900.0), 0);
+        assert_eq!(drop_index(890.0, 2, 900.0), 2);
+    }
+
     #[test]
     fn len_reports_stack_size() {
         let state = DockState::default();