Display list to Metal render passes #118

Phase 12: Metal GPU Compositor (5/7)#

Goal: Translate the existing PaintCommand display list into Metal GPU draw calls, replacing the software renderer for the main rendering path.

Depends on: Glyph texture atlas for GPU text rendering

Requirements#

• Implement display list → Metal translation for each PaintCommand variant:

  • FillRect → emit a solid-color quad (4 vertices, 2 triangles)
  • DrawGlyphs → for each glyph in the TextLine, emit a textured quad sampling from the glyph atlas, tinted with the text color
  • DrawImage → emit a textured quad sampling from the image's MTLTexture
  • PushClip → set MTLRenderCommandEncoder scissor rect (intersect with current)
  • PopClip → restore previous scissor rect from stack

• Batch draw calls for efficiency:

  • Collect all solid-color quads into one vertex buffer range
  • Collect all textured quads (same atlas page) into another range
  • Minimize draw call count by batching quads that share the same texture
  • Maintain correct painter's order (don't reorder across clip boundaries)

• Handle scroll offsets:

  • Apply page scroll offset as a vertex transform (translate all positions by -scroll_y)
  • Per-element scroll offsets applied during display list generation (already handled)

• Wire into browser main loop:

  • render_page() produces display list → Metal backend processes it → presents drawable
  • Replace the current Renderer → BitmapContext → CGImage path

Acceptance Criteria#

• All PaintCommand types render correctly via Metal
• Clipping (scissor rects) works for overflow containers
• Visual output matches the software renderer for test pages
• Scrolling works correctly
• Draw calls are batched (not one draw per rectangle)
• No external crate dependencies
• cargo clippy --workspace -- -D warnings passes
• cargo test --workspace passes