Extract Canvas draw_merge layer blend helper

docs/modernization/debt.md

@@ -18,6 +18,13 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Recent Reductions
 
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge()` per-layer
+  blend composite now routes merge-RTT unbind, shader setup, optional
+  destination-copy feedback, draw, and cleanup ordering through
+  `execute_legacy_canvas_draw_merge_layer_blend(...)`; layer iteration,
+  temporary-stroke branch selection, framebuffer copies, sampler/texture
+  object callbacks, and final merged-plane ownership remain retained in
+  `Canvas`.
 - 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge()` erase live
   temporary-stroke composite now routes retained setup, sampler bind, texture
   bind, draw, and texture unbind ordering through

docs/modernization/roadmap.md

@@ -3145,6 +3145,11 @@ Results:
   `execute_legacy_canvas_stroke_temporary_composite(...)`, leaving only the
   concrete GL object callbacks and broader final composite ownership in the
   legacy Canvas path.
+- `Canvas::draw_merge()` per-layer blend composite now routes merge-RTT
+  unbind, shader setup, optional destination-copy feedback, draw, and cleanup
+  ordering through `execute_legacy_canvas_draw_merge_layer_blend(...)`, while
+  temporary-stroke branch selection, framebuffer copies, and final merged-plane
+  ownership remain in the legacy Canvas path.
 - `NodeStrokePreview::draw_stroke_immediate()` now shares
   `execute_legacy_node_stroke_preview_pass_sequence(...)` for
   dual-pass/background/main-pass/final-composite/copy-back ordering, while the

docs/modernization/tasks.md

@@ -509,6 +509,15 @@ Done Checks:
 
 Progress Notes:
 
+- 2026-06-13: `Canvas::draw_merge()` per-layer blend composite now routes
+  merge-RTT unbind, shader setup, optional destination-copy feedback, draw,
+  and cleanup ordering through
+  `execute_legacy_canvas_draw_merge_layer_blend(...)`; temporary-stroke
+  branch selection, layer iteration, and final merged-plane redraw ownership
+  remain local to `Canvas`. Next slice should target the remaining end-of-plane
+  merged-texture copy/grid/final-redraw seam or another similarly narrow final
+  composite boundary without reopening landed temporary-composite or
+  per-layer blend helpers.
 - 2026-06-13: `Canvas::draw_merge()` erase live temporary-stroke composite now
   routes retained setup, sampler bind, texture bind, draw, and texture unbind
   ordering through `execute_legacy_canvas_stroke_temporary_composite(...)`;

src/canvas.cpp

@@ -1535,47 +1535,49 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
 
             if (use_blend)
             {
-                m_merge_rtt.unbindFramebuffer();
+                pp::panopainter::execute_legacy_canvas_draw_merge_layer_blend(
-            }
+                    pp::panopainter::LegacyCanvasDrawMergeLayerBlendUniforms {
-
+                        .shader = {
-            // draw the blended
+                            .mvp = ortho,
-            if (use_blend)
+                            .texture_slot = 0,
-            {
+                            .destination_texture_slot = 2,
-                m_sampler.bind(0);
+                            .use_destination_texture = copy_blend_destination,
-                m_sampler.bind(2);
+                            .blend_mode = m_layers[layer_index]->m_blend_mode,
-
+                            .alpha = 1.f,
-                pp::panopainter::setup_legacy_canvas_draw_merge_texture_blend_shader(
+                        },
-                    pp::panopainter::LegacyCanvasDrawMergeTextureBlendUniforms {
+                        .copy_destination = copy_blend_destination,
-                        .mvp = ortho,
+                    },
-                        .texture_slot = 0,
+                    pp::panopainter::LegacyCanvasDrawMergeLayerBlendExecution {
-                        .destination_texture_slot = 2,
+                        .unbind_merge_framebuffer = [&] {
-                        .use_destination_texture = copy_blend_destination,
+                            m_merge_rtt.unbindFramebuffer();
-                        .blend_mode = m_layers[layer_index]->m_blend_mode,
+                        },
-                        .alpha = 1.f,
+                        .bind_samplers = [&] {
+                            m_sampler.bind(0);
+                            m_sampler.bind(2);
+                        },
+                        .bind_merge_texture = [&] {
+                            set_active_texture_unit(0);
+                            m_merge_rtt.bindTexture();
+                        },
+                        .bind_destination_texture = [&] {
+                            set_active_texture_unit(2);
+                            m_merge_tex.bind();
+                        },
+                        .copy_destination_framebuffer = [&] {
+                            copy_framebuffer_to_texture_2d(0, 0, 0, 0, m_width, m_height);
+                        },
+                        .draw = [&] {
+                            m_plane.draw_fill();
+                        },
+                        .unbind_destination_texture = [&] {
+                            set_active_texture_unit(2);
+                            m_merge_tex.unbind();
+                        },
+                        .unbind_merge_texture = [&] {
+                            set_active_texture_unit(0);
+                            m_merge_rtt.unbindTexture();
+                        },
                     });
-                if (copy_blend_destination)
-                {
-                    m_sampler.bind(2);
-                }
-
-                set_active_texture_unit(0);
-                m_merge_rtt.bindTexture();
-                if (copy_blend_destination)
-                {
-                    set_active_texture_unit(2);
-                    m_merge_tex.bind();
-                    copy_framebuffer_to_texture_2d(0, 0, 0, 0, m_width, m_height);
-                }
-
-                m_plane.draw_fill();
-
-                if (copy_blend_destination)
-                {
-                    set_active_texture_unit(2);
-                    m_merge_tex.unbind();
-                }
-                set_active_texture_unit(0);
-                m_merge_rtt.unbindTexture();
             }
         }
 

src/legacy_canvas_draw_merge_services.h

@@ -64,6 +64,22 @@ struct LegacyCanvasDrawMergeShaderExecution {
     std::function<void(kShaderUniform, const glm::mat4&)> set_mat4;
 };
 
+struct LegacyCanvasDrawMergeLayerBlendUniforms {
+    LegacyCanvasDrawMergeTextureBlendUniforms shader;
+    bool copy_destination = false;
+};
+
+struct LegacyCanvasDrawMergeLayerBlendExecution {
+    std::function<void()> unbind_merge_framebuffer;
+    std::function<void()> bind_samplers;
+    std::function<void()> bind_merge_texture;
+    std::function<void()> bind_destination_texture;
+    std::function<void()> copy_destination_framebuffer;
+    std::function<void()> draw;
+    std::function<void()> unbind_destination_texture;
+    std::function<void()> unbind_merge_texture;
+};
+
 [[nodiscard]] inline LegacyCanvasDrawMergeShaderExecution legacy_shader_manager_draw_merge_execution() noexcept
 {
     return {
@@ -218,4 +234,27 @@ inline void setup_legacy_canvas_draw_merge_texture_colorize_shader(
     setup_legacy_canvas_draw_merge_texture_colorize_shader(uniforms, legacy_shader_manager_draw_merge_execution());
 }
 
+inline void execute_legacy_canvas_draw_merge_layer_blend(
+    const LegacyCanvasDrawMergeLayerBlendUniforms& uniforms,
+    const LegacyCanvasDrawMergeLayerBlendExecution& execution)
+{
+    execution.unbind_merge_framebuffer();
+    execution.bind_samplers();
+    setup_legacy_canvas_draw_merge_texture_blend_shader(uniforms.shader);
+    execution.bind_merge_texture();
+
+    if (uniforms.copy_destination) {
+        execution.bind_destination_texture();
+        execution.copy_destination_framebuffer();
+    }
+
+    execution.draw();
+
+    if (uniforms.copy_destination) {
+        execution.unbind_destination_texture();
+    }
+
+    execution.unbind_merge_texture();
+}
+
 } // namespace pp::panopainter