Stage 2 — Outputs & Render Loop

What it is. Moving from “clear the screen once” (Stage 0) to a real, continuous render loop driven by wlroots’ scene graph — the data structure that holds everything that gets drawn and knows how to repaint only what changed (damage tracking).

Why it matters. Every window, background, and layer-shell surface added in later stages is a node in this scene graph. Getting the output/scene/ layout wiring right here is what lets later stages just add nodes instead of hand-rolling their own render passes.

Deliverable (from KICKOFF.md): wlr_output, wlr_scene, per-frame render with damage. A stable, damage-tracked frame on nested + headless.

How it went

Three pieces get created once in main() and tied together: wlr_scene_create (the scene graph itself), wlr_output_layout_create (where outputs sit in space), and wlr_scene_attach_output_layout, which keeps a scene-output positioned to match its layout slot automatically. Each new output (handled in handle_new_output, see src/output.rs ) gets a wlr_scene_output tied to a layout slot via wlr_scene_output_layout_add_output, and a frame listener that calls wlr_scene_output_render on every frame the output signals it’s ready for one.

The scene is organized as an ordered stack of layer trees — direct children of the scene root, created in a fixed order so creation order becomes paint order (background → layer-shell bottom → normal app windows → layer-shell top → layer-shell overlay). That ordering, decided here, is what Stage 8’s layer-shell support slots into later without needing to touch the scene wiring again.

Status: done, and the scene/output/layout structure from this stage is unchanged in shape today — later stages extended it (per-output tiling in Stage 5, forced repaint-on-resume in Stage 6) rather than replacing it.