SystemManagers.Initialize() Registration Contract

Each render backend has its own SystemManagers (not shared — see gum-runtime-topology for why RenderingLibrary.* compiles into multiple assemblies). Its Initialize()/Initialize(fullInstantiation: true) is what makes that backend's renderables visible to Gum's primary runtime-type registry. There is no shared interface or base-class checklist for this — each backend hand-writes the same sequence. New backend implementers (e.g. Silk.NET, #2738) must replicate it.

The contract

A conforming Initialize() must, in some order:

1. LoaderManager.Self.ContentLoader = new <Backend>ContentLoader(); — wires asset loading (textures, fonts).
2. GraphicalUiElement.SetPropertyOnRenderable = CustomSetPropertyOnRenderable.SetPropertyOnRenderable; (+ usually AddRenderableToManagers, RemoveRenderableFromManagers, UpdateFontFromProperties) — routes .gumx property assignments into the backend's renderables.
3. ElementSaveExtensions.CustomCreateGraphicalComponentFunc = <backend's factory>; — the backend's own IRenderable factory, tried by the fallback path (see below) before FallbackRenderableFactory.
4. StandardElementsManager.Self.Initialize(); — registers default states for the standard element types.
5. Register the 8 standard shape types via ElementSaveExtensions.RegisterGueInstantiation(name, () => new XRuntime()): Circle, ColoredRectangle, Container, NineSlice, Polygon, Rectangle, Sprite, Text. This is the primary registry Gum uses to build the GraphicalUiElement visual tree — see the two-registry model documented at the top of Gum/Wireframe/FallbackRenderableFactory.cs.

Reference implementations: RenderingLibrary/SystemManagers.cs:139-294 (MonoGame/KNI/FNA, the most complete — also loads embedded default fonts and installs a mobile/browser file-stream hook), Runtimes/RaylibGum/RenderingLibrary/SystemManagers.cs:115-145, Runtimes/SkiaGum/RenderingLibrary/SystemManagers.cs:55-91, Runtimes/SokolGum/SystemManagers.cs:65-137.

Landmines

• A missing shape registration doesn't throw — it silently falls back. If step 5 skips a type, ElementSaveExtensions' primary registry has nothing for it, so Gum falls through to FallbackRenderableFactory.TryHandleAsBaseType (Gum/Wireframe/FallbackRenderableFactory.cs), a hardcoded per-backend switch that returns a plainer renderable (e.g. an outline-only LineCircle instead of the backend's preferred filled shape). This is how a backend can look like it "supports" a shape while actually degrading it — there's no compile-time or startup check that all 8 are registered. Don't extend FallbackRenderableFactory's switch to paper over a missing registration in a new backend; register the real runtime wrapper in step 5 instead.
• These are process-wide static assignments, not instance state — ElementSaveExtensions.CustomCreateGraphicalComponentFunc, GraphicalUiElement.SetPropertyOnRenderable, LoaderManager.Self.ContentLoader, etc. are shared statics every backend's Initialize() overwrites. Running two backends (or two SystemManagers instances of different backends) in one process means the last Initialize() call wins — intentional for the typical single-backend app, but not safe to assume otherwise.
• Extended shape types beyond the standard 8 need their own wiring, not just step 5. Skia additionally registers Arc, ColoredCircle, Line, and handles RoundedRectangle — these aren't part of StandardElementsManager's default-state set, so Skia also assigns StandardElementsManager.Self.CustomGetDefaultState (Runtimes/SkiaGum/RenderingLibrary/SystemManagers.cs:78,190-206) to supply default states for them. Only copy this if the new backend adds shape types beyond the standard 8.
• Idempotency guards vary and matter. Skia guards its whole registration block behind HasInitializedGlobal (Runtimes/SkiaGum/RenderingLibrary/SystemManagers.cs:72-88) because it can be constructed per-window; re-running the block wipes customizations of standard elements. MonoGame/Raylib/Sokol don't guard and expect Initialize() to run once per process. Decide which fits the new backend's construction pattern rather than copying one blindly.