This is a Unity Editor extension package that injects custom GUI into Unity's toolbar via reflection. The core library (ToolbarExtender.cs) exposes two static lists of Action delegates (LeftToolbarGUI and RightToolbarGUI) that users populate in InitializeOnLoad static constructors. The companion ToolbarCallback.cs handles the low-level reflection hooking, with version-specific code paths for Unity 2018 through Unity 6.3 (which uses UI Toolkit docks instead of IMGUI). Two example implementations demonstrate usage: a scene switcher with numbered buttons and a scene view focuser toggle that persists state via EditorPrefs. The architecture is minimal but effective; the library has no external dependencies beyond the Unity Editor API and is distributed as a UPM package. The code is well-structured for its niche, though it remains inherently fragile due to its reliance on Unity's internal API reflection.