Route NodeCanvas blend setup through helper

docs/modernization/debt.md

@@ -18,6 +18,12 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Recent Reductions
 
+- 2026-06-13: DEBT-0036 was narrowed again. `NodeCanvas` non-stroke
+  `TextureBlend` shader setup now routes through
+  `legacy_canvas_draw_merge_services.h`, including the optional destination
+  texture uniform only when blend destination copies are used. NodeCanvas still
+  owns panorama layer traversal, onion-frame drawing, blend destination copies,
+  sampler/texture binding, framebuffer lifetime, and draw ordering.
 - 2026-06-13: DEBT-0036 was narrowed again. `NodeCanvas` now reuses
   `legacy_canvas_draw_merge_services.h` for retained checkerboard grid shader
   setup and the final cached-layer texture redraw setup. NodeCanvas still owns

docs/modernization/roadmap.md

@@ -3000,6 +3000,11 @@ Results:
   setup helper for checkerboard grids and final cached-layer texture redraws;
   onion-frame traversal, blend copies, texture binding, and draw order remain
   retained in the node.
+- `NodeCanvas` non-stroke layer blending now shares the retained draw-merge
+  shader setup helper for `TextureBlend` uniforms, preserving the optional
+  destination texture uniform when copy-based blending is active. Framebuffer
+  lifetime, destination copies, texture binding, and draw execution remain
+  local to the node.
 - Canvas thumbnail layer blending now uses the same canvas destination-feedback
   plan for framebuffer-fetch versus texture-copy decisions; the thumbnail draw
   itself still executes through retained OpenGL canvas code under DEBT-0036.

src/legacy_canvas_draw_merge_services.h

@@ -16,9 +16,19 @@ struct LegacyCanvasDrawMergeTextureUniforms {
     int texture_slot = 0;
 };
 
+struct LegacyCanvasDrawMergeTextureBlendUniforms {
+    glm::mat4 mvp { 1.0f };
+    int texture_slot = 0;
+    int destination_texture_slot = 2;
+    bool use_destination_texture = false;
+    int blend_mode = 0;
+    float alpha = 1.0f;
+};
+
 struct LegacyCanvasDrawMergeShaderExecution {
     std::function<void(kShader)> use_shader;
     std::function<void(kShaderUniform, int)> set_int;
+    std::function<void(kShaderUniform, float)> set_float;
     std::function<void(kShaderUniform, const glm::mat4&)> set_mat4;
 };
 
@@ -27,6 +37,7 @@ struct LegacyCanvasDrawMergeShaderExecution {
     return {
         .use_shader = [](kShader shader) { ShaderManager::use(shader); },
         .set_int = [](kShaderUniform uniform, int value) { ShaderManager::u_int(uniform, value); },
+        .set_float = [](kShaderUniform uniform, float value) { ShaderManager::u_float(uniform, value); },
         .set_mat4 = [](kShaderUniform uniform, const glm::mat4& value) { ShaderManager::u_mat4(uniform, value); },
     };
 }
@@ -61,4 +72,24 @@ inline void setup_legacy_canvas_draw_merge_texture_shader(
     setup_legacy_canvas_draw_merge_texture_shader(uniforms, legacy_shader_manager_draw_merge_execution());
 }
 
+inline void setup_legacy_canvas_draw_merge_texture_blend_shader(
+    const LegacyCanvasDrawMergeTextureBlendUniforms& uniforms,
+    const LegacyCanvasDrawMergeShaderExecution& execution) noexcept
+{
+    execution.use_shader(kShader::TextureBlend);
+    execution.set_int(kShaderUniform::Tex, uniforms.texture_slot);
+    if (uniforms.use_destination_texture) {
+        execution.set_int(kShaderUniform::TexBG, uniforms.destination_texture_slot);
+    }
+    execution.set_int(kShaderUniform::BlendMode, uniforms.blend_mode);
+    execution.set_float(kShaderUniform::Alpha, uniforms.alpha);
+    execution.set_mat4(kShaderUniform::MVP, uniforms.mvp);
+}
+
+inline void setup_legacy_canvas_draw_merge_texture_blend_shader(
+    const LegacyCanvasDrawMergeTextureBlendUniforms& uniforms)
+{
+    setup_legacy_canvas_draw_merge_texture_blend_shader(uniforms, legacy_shader_manager_draw_merge_execution());
+}
+
 } // namespace pp::panopainter

src/node_canvas.cpp

@@ -606,13 +606,15 @@ void NodeCanvas::draw()
                     m_sampler.bind(0);
                     m_sampler.bind(2);
 
-                    ShaderManager::use(kShader::TextureBlend);
+                    pp::panopainter::setup_legacy_canvas_draw_merge_texture_blend_shader(
-                    ShaderManager::u_int(kShaderUniform::Tex, 0);
+                        pp::panopainter::LegacyCanvasDrawMergeTextureBlendUniforms {
-                    if (copy_blend_destination)
+                            .mvp = glm::ortho(-1, 1, -1, 1),
-                        ShaderManager::u_int(kShaderUniform::TexBG, 2);
+                            .texture_slot = 0,
-                    ShaderManager::u_int(kShaderUniform::BlendMode, m_canvas->m_layers[layer_index]->m_blend_mode);
+                            .destination_texture_slot = 2,
-                    ShaderManager::u_float(kShaderUniform::Alpha, 1.f);
+                            .use_destination_texture = copy_blend_destination,
-                    ShaderManager::u_mat4(kShaderUniform::MVP, glm::ortho(-1, 1, -1, 1));
+                            .blend_mode = m_canvas->m_layers[layer_index]->m_blend_mode,
+                            .alpha = 1.f,
+                        });
                 
                     set_active_texture_unit(0);
                     m_blender_rtt.bindTexture();