Date: 2026-03-01
Issue: #180
Domain: Runtime-viewer bridge precondition for egui_glow -> egui_wgpu

Capture the current baseline before implementing the GL->wgpu bridge spike. This establishes measurable starting facts and narrows the first executable spike surface.

1. egui_wgpu is not yet present in dependencies.
   • Evidence: Cargo.toml currently includes egui_glow and no egui_wgpu entry.
2. Composited content callback path is GL-bound through Servo offscreen contexts.
   • Evidence hotspot: shell/desktop/workbench/compositor_adapter.rs
   • Key seam: render_context.render_to_parent_callback() in register_content_pass_from_render_context(...).
3. Runtime viewer composition currently depends on OffscreenRenderingContext across workbench/UI lifecycle modules.
   • Hotspots include:
     • shell/desktop/workbench/tile_compositor.rs
     • shell/desktop/workbench/tile_render_pass.rs
     • shell/desktop/workbench/tile_runtime.rs
     • shell/desktop/ui/gui.rs
     • shell/desktop/ui/gui_frame.rs

Define the first spike as one composited content path only:

• Keep existing egui_glow mainline untouched.
• Add an isolated wgpu-spike module behind feature gate and/or test harness entry.
• Target one node-pane content surface and measure:
  • copy count
  • callback latency
  • frame-time delta
  • resize behavior
  • repeated tile-rect update behavior

For each test run, capture:

• bridge path used (copy/interop strategy)
• tile rect + render size
• end-to-end callback time (us)
• presentation time (us)
• dropped/failed frame count

Implement a minimal bridge-spike harness that can run without replacing the active renderer backend, then record measured results in a follow-up receipt.

#183 remains blocked until this spike provides measured bridge viability evidence.