UI

Layer 2. Logical 2D UI — widget tree, layout, hit-testing, event dispatch, font atlas. The GPU side lives in mjolnir/render/ui/ and is driven by the engine.

Widgets

Four kinds, all sharing a WidgetBase (position, z-order, visible flag, parent, event handlers, user-data pointer):

• Mesh2D — arbitrary triangle list, for shapes the other primitives can’t express.
• Quad2D — colored or textured rectangle. The workhorse.
• Text2D — fontstash-laid-out glyph quads with horizontal + vertical alignment inside a bounds box.
• Box — container with children; the parent-relative coordinate origin for everything beneath it.

Each widget kind has its own distinct handle type (Quad2DHandle, …) on top of UIWidgetHandle for compile-time safety.

Layout

Layout is parent-relative + z-ordered. compute_layout_all resolves every Box subtree once per frame after user mutations; the engine calls it before generating draw commands.

Z-ordering uses a stable integer rather than insertion order so overlays, modals, and HUD layers can be set explicitly without re-parenting.

Events

The engine’s input loop calls pick_widget(point) each frame, then dispatch_mouse_event with the resulting handle. The picker walks the widget tree top-down by z-order and returns the first widget whose bounds contain the cursor.

Hover transitions (HOVER_IN / HOVER_OUT) are derived by tracking the previous-frame hovered widget. Click events bubble parent-ward by default so a click on a child of a Box also fires on the Box unless the child stops propagation.

Text

Fontstash backs glyph rasterization into a single atlas texture managed by the UI system. Glyphs are cached on first use; the engine calls update_font_atlas once per frame to upload any new glyphs to the GPU.