sigc::mem_fun already disconnects the callbacks bound to any instance of sigc::trackable. So the real problem is that the Bar doesn't inherit from trackable.

diff --git a/include/bar.hpp b/include/bar.hpp
index 27bf877d..d079e04c 100644
--- a/include/bar.hpp
+++ b/include/bar.hpp
@@ -54,7 +54,7 @@ class BarIpcClient;
 }
 #endif  // HAVE_SWAY
 
-class Bar {
+class Bar : public sigc::trackable {
  public:
   using bar_mode_map = std::map<std::string, struct bar_mode>;
   static const bar_mode_map PRESET_MODES;

sigc::mem_fun already disconnects the callbacks bound to any instance of sigc::trackable. So the real problem is that the Bar doesn't inherit from trackable.

diff --git a/include/bar.hpp b/include/bar.hpp
index 27bf877d..d079e04c 100644
--- a/include/bar.hpp
+++ b/include/bar.hpp
@@ -54,7 +54,7 @@ class BarIpcClient;
 }
 #endif  // HAVE_SWAY
 
-class Bar {
+class Bar : public sigc::trackable {
  public:
   using bar_mode_map = std::map<std::string, struct bar_mode>;
   static const bar_mode_map PRESET_MODES;

Yep, I did not notice that it uses sigc::mem_fun(), I thought it was using std::mem_fun().

Will update and test later.