Extract Canvas draw_merge final plane composite helper

docs/modernization/debt.md

@@ -301,6 +301,12 @@ agent or engineer to remove them without reconstructing context from chat.
   through `legacy_canvas_draw_merge_services.h`. The retained Canvas path still
   owns draw-merge layer iteration, blend-gate branching, framebuffer copies,
   sampler/texture binding, and draw ordering.
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge` end-of-plane
+  merged-texture copy, optional checkerboard redraw, and final merged-texture
+  composite ordering now route through
+  `execute_legacy_canvas_draw_merge_final_plane_composite(...)`; the retained
+  Canvas path still owns per-plane iteration plus the concrete framebuffer,
+  sampler, texture, and draw callbacks.
 - 2026-06-13: DEBT-0036 was narrowed again. `Canvas::layer_merge` now reuses
   `legacy_canvas_stroke_composite_services.h` for retained layer-merge
   `kShader::CompDraw` binding and source/destination blend uniform writes.

docs/modernization/roadmap.md

@@ -3150,6 +3150,11 @@ Results:
   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.
+- `Canvas::draw_merge()` end-of-plane merged-texture copy plus optional
+  checkerboard/final-texture redraw ordering now routes through
+  `execute_legacy_canvas_draw_merge_final_plane_composite(...)`, while
+  per-plane iteration and the concrete framebuffer, sampler, texture, and draw
+  callbacks 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

@@ -528,6 +528,13 @@ Progress Notes:
   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()` end-of-plane merged-texture copy,
+  optional checkerboard redraw, and final merged-texture composite ordering now
+  route through `execute_legacy_canvas_draw_merge_final_plane_composite(...)`;
+  per-plane iteration and concrete framebuffer, sampler, texture, and draw
+  callbacks remain local to `Canvas`. Next slice should target another narrow
+  retained draw-merge boundary without reopening landed temporary-composite,
+  layer-blend, or final-plane 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

@@ -1581,37 +1581,45 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
             }
         }
 
-        set_active_texture_unit(2);
-        m_merge_tex.bind();
-        copy_framebuffer_to_texture_2d(0, 0, 0, 0, m_width, m_height);
-        
-        // draw the grid behind the layers using a temporary copy
         if (use_blend)
         {
-            apply_canvas_capability(blend_state(), true);
-
-            //draw the grid
-            if (draw_checkerboard)
-            {
-                pp::panopainter::setup_legacy_canvas_draw_merge_checkerboard_shader(
-                    pp::panopainter::LegacyCanvasDrawMergeCheckerboardUniforms {
+            pp::panopainter::execute_legacy_canvas_draw_merge_final_plane_composite(
+                pp::panopainter::LegacyCanvasDrawMergeFinalPlaneCompositeUniforms {
+                    .checkerboard = {
                         .mvp = ortho,
                         .colorize = false,
-                    });
-                m_plane.draw_fill();
-            }
-
-            // draw the layers
-            m_sampler.bind(0);
-            set_active_texture_unit(0);
-            m_merge_tex.bind();
-            pp::panopainter::setup_legacy_canvas_draw_merge_texture_shader(
-                pp::panopainter::LegacyCanvasDrawMergeTextureUniforms {
-                    .mvp = ortho,
-                    .texture_slot = 0,
+                    },
+                    .texture = {
+                        .mvp = ortho,
+                        .texture_slot = 0,
+                    },
+                    .draw_checkerboard = draw_checkerboard,
+                },
+                pp::panopainter::LegacyCanvasDrawMergeFinalPlaneCompositeExecution {
+                    .bind_merged_texture_copy_target = [&] {
+                        set_active_texture_unit(2);
+                        m_merge_tex.bind();
+                    },
+                    .copy_merged_framebuffer = [&] {
+                        copy_framebuffer_to_texture_2d(0, 0, 0, 0, m_width, m_height);
+                    },
+                    .enable_blend = [&] {
+                        apply_canvas_capability(blend_state(), true);
+                    },
+                    .draw = [&] {
+                        m_plane.draw_fill();
+                    },
+                    .bind_sampler = [&] {
+                        m_sampler.bind(0);
+                    },
+                    .bind_merged_texture = [&] {
+                        set_active_texture_unit(0);
+                        m_merge_tex.bind();
+                    },
+                    .unbind_merged_texture = [&] {
+                        m_merge_tex.unbind();
+                    },
                 });
-            m_plane.draw_fill();
-            m_merge_tex.unbind();
         }
         
         m_layers_merge.rtt(plane_index).unbindFramebuffer();

src/legacy_canvas_draw_merge_services.h

@@ -80,6 +80,22 @@ struct LegacyCanvasDrawMergeLayerBlendExecution {
     std::function<void()> unbind_merge_texture;
 };
 
+struct LegacyCanvasDrawMergeFinalPlaneCompositeUniforms {
+    LegacyCanvasDrawMergeCheckerboardUniforms checkerboard;
+    LegacyCanvasDrawMergeTextureUniforms texture;
+    bool draw_checkerboard = false;
+};
+
+struct LegacyCanvasDrawMergeFinalPlaneCompositeExecution {
+    std::function<void()> bind_merged_texture_copy_target;
+    std::function<void()> copy_merged_framebuffer;
+    std::function<void()> enable_blend;
+    std::function<void()> draw;
+    std::function<void()> bind_sampler;
+    std::function<void()> bind_merged_texture;
+    std::function<void()> unbind_merged_texture;
+};
+
 [[nodiscard]] inline LegacyCanvasDrawMergeShaderExecution legacy_shader_manager_draw_merge_execution() noexcept
 {
     return {
@@ -257,4 +273,25 @@ inline void execute_legacy_canvas_draw_merge_layer_blend(
     execution.unbind_merge_texture();
 }
 
+inline void execute_legacy_canvas_draw_merge_final_plane_composite(
+    const LegacyCanvasDrawMergeFinalPlaneCompositeUniforms& uniforms,
+    const LegacyCanvasDrawMergeFinalPlaneCompositeExecution& execution)
+{
+    execution.bind_merged_texture_copy_target();
+    execution.copy_merged_framebuffer();
+
+    execution.enable_blend();
+
+    if (uniforms.draw_checkerboard) {
+        setup_legacy_canvas_draw_merge_checkerboard_shader(uniforms.checkerboard);
+        execution.draw();
+    }
+
+    execution.bind_sampler();
+    execution.bind_merged_texture();
+    setup_legacy_canvas_draw_merge_texture_shader(uniforms.texture);
+    execution.draw();
+    execution.unbind_merged_texture();
+}
+
 } // namespace pp::panopainter