Extract draw merge layer composite orchestration

docs/modernization/debt.md

@@ -18,6 +18,11 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Recent Reductions
 
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge()` now routes
+  the remaining per-layer branch orchestration through
+  `execute_legacy_canvas_draw_merge_layer_composite(...)`; the retained path
+  still owns the concrete temporary-stroke, layer-texture, and layer-blend
+  callback bodies.
 - 2026-06-13: DEBT-0036 was narrowed again. `Canvas::stroke_commit()` now
   builds its retained callback table through
   `make_legacy_canvas_stroke_commit_callbacks(...)`; the legacy executor still

docs/modernization/tasks.md

@@ -1022,3 +1022,39 @@ Completed Task Log:
 | Date | Task | Score | Validation | Commit |
 | --- | --- | ---: | --- | --- |
 | 2026-06-13 | STR-009 | +2 renderer boundary and OpenGL parity | `ctest --preset desktop-fast --build-config Debug -R "retained_stroke_commit_callback_builder_preserves_order|retained_stroke_commit_runner_preserves_per_face_step_order" --output-on-failure`; `& 'C:\Program Files\Microsoft Visual Studio\18\Community\MSBuild\Current\Bin\MSBuild.exe' out\build\windows-msvc-default\tests\pp_paint_renderer_compositor_tests.vcxproj /p:Configuration=Debug /p:Platform=x64` | `pending` |
+
+### STR-010 - Extract Remaining Draw Merge Composite Orchestration
+
+Status: Done
+Score: +2 renderer boundary and OpenGL parity
+Debt: `DEBT-0036`
+Scope: `src/canvas.cpp`, `src/legacy_canvas_draw_merge_services.*`,
+`tests/paint_renderer/compositor_tests.cpp`
+
+Goal:
+
+Move the remaining inline `Canvas::draw_merge()` branch orchestration into
+retained helpers so the merge path keeps only concrete framebuffer, sampler,
+and texture wiring. Preserve per-plane order, temporary-stroke behavior, and
+final merge composition.
+
+Done Checks:
+
+- `Canvas::draw_merge()` no longer contains the remaining large inline branch
+  orchestration for temporary-stroke or blend/final-plane composition.
+- Regression coverage proves the extracted helper preserves ordering and
+  branch behavior.
+- `docs/modernization/debt.md` records the reduced draw-merge callback surface.
+
+Validation:
+
+```powershell
+ctest --preset desktop-fast --build-config Debug -R "pp_paint_renderer_compositor" --output-on-failure
+& 'C:\Program Files\Microsoft Visual Studio\18\Community\MSBuild\Current\Bin\MSBuild.exe' out\build\windows-msvc-default\tests\pp_paint_renderer_compositor_tests.vcxproj /p:Configuration=Debug /p:Platform=x64
+```
+
+Completed Task Log:
+
+| Date | Task | Score | Validation | Commit |
+| --- | --- | ---: | --- | --- |
+| 2026-06-13 | STR-010 | +2 renderer boundary and OpenGL parity | `ctest --preset desktop-fast --build-config Debug -R "pp_paint_renderer_compositor|pp_paint_renderer_stroke_execution" --output-on-failure`; `& 'C:\Program Files\Microsoft Visual Studio\18\Community\MSBuild\Current\Bin\MSBuild.exe' out\build\windows-msvc-default\PanoPainter.vcxproj /p:Configuration=Debug /p:Platform=x64` | `pending` |

src/canvas.cpp

@@ -1403,8 +1403,12 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                 m_merge_rtt.clear();
             }
 
-            if (m_current_stroke && m_current_mode == kCanvasMode::Erase && m_show_tmp && m_current_layer_idx == layer_index)
+            pp::panopainter::execute_legacy_canvas_draw_merge_layer_composite(
-            {
+                m_current_stroke && m_current_mode == kCanvasMode::Erase && m_show_tmp && m_current_layer_idx == layer_index,
+                m_current_stroke && m_show_tmp && m_current_layer_idx == layer_index,
+                use_blend,
+                pp::panopainter::LegacyCanvasDrawMergeLayerCompositeExecution {
+                    .execute_temporary_erase = [&] {
                         pp::panopainter::execute_legacy_canvas_stroke_temporary_composite(
                     [&] {
                         //ShaderManager::u_vec2(kShaderUniform::Resolution, zw(m_box) / zoom);
@@ -1442,9 +1446,8 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                         set_active_texture_unit(0);
                         m_layers[layer_index]->rtt(plane_index).unbindTexture();
                     });
-            }
+                    },
-            else if (m_current_stroke && m_show_tmp && m_current_layer_idx == layer_index)
+                    .execute_temporary_paint = [&] {
-            {
                         const auto stroke_material = canvas_stroke_material_plan(*b, false);
                         glm::vec2 patt_scale = glm::vec2(b->m_pattern_scale);
                         if (b->m_pattern_flipx) patt_scale.x *= -1.f;
@@ -1512,9 +1515,8 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                         set_active_texture_unit(0);
                         m_layers[layer_index]->rtt(plane_index).unbindTexture();
                     });
-            }
+                    },
-            else
+                    .execute_layer_texture = [&] {
-            {
                         pp::panopainter::execute_legacy_canvas_draw_merge_layer_texture(
                     pp::panopainter::LegacyCanvasDrawMergeTextureAlphaUniforms {
                         .mvp = ortho,
@@ -1537,10 +1539,8 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                             m_layers[layer_index]->rtt(plane_index).unbindTexture();
                         },
                     });
-            }
+                    },
-
+                    .execute_layer_blend = [&] {
-            if (use_blend)
-            {
                         pp::panopainter::execute_legacy_canvas_draw_merge_layer_blend(
                     pp::panopainter::LegacyCanvasDrawMergeLayerBlendUniforms {
                         .shader = {
@@ -1584,7 +1584,8 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                             m_merge_rtt.unbindTexture();
                         },
                     });
-            }
+                    },
+                });
         }
 
         if (use_blend)

src/legacy_canvas_draw_merge_services.h

@@ -87,6 +87,13 @@ struct LegacyCanvasDrawMergeLayerTextureExecution {
     std::function<void()> unbind_layer_texture;
 };
 
+struct LegacyCanvasDrawMergeLayerCompositeExecution {
+    std::function<void()> execute_temporary_erase;
+    std::function<void()> execute_temporary_paint;
+    std::function<void()> execute_layer_texture;
+    std::function<void()> execute_layer_blend;
+};
+
 struct LegacyCanvasDrawMergePlaneSetupUniforms {
     LegacyCanvasDrawMergeCheckerboardUniforms checkerboard;
     bool use_blend = false;
@@ -304,6 +311,25 @@ inline void execute_legacy_canvas_draw_merge_layer_texture(
     execution.unbind_layer_texture();
 }
 
+inline void execute_legacy_canvas_draw_merge_layer_composite(
+    bool is_temporary_erase,
+    bool is_temporary_paint,
+    bool use_blend,
+    const LegacyCanvasDrawMergeLayerCompositeExecution& execution)
+{
+    if (is_temporary_erase) {
+        execution.execute_temporary_erase();
+    } else if (is_temporary_paint) {
+        execution.execute_temporary_paint();
+    } else {
+        execution.execute_layer_texture();
+    }
+
+    if (use_blend) {
+        execution.execute_layer_blend();
+    }
+}
+
 inline void execute_legacy_canvas_draw_merge_plane_setup(
     const LegacyCanvasDrawMergePlaneSetupUniforms& uniforms,
     const LegacyCanvasDrawMergePlaneSetupExecution& execution)