Extract draw merge temporary branch helper

docs/modernization/debt.md

@@ -80,6 +80,11 @@ agent or engineer to remove them without reconstructing context from chat.
   composite setup now routes through `execute_canvas_draw_merge_temporary_composite(...)`;
   setup, sampler, texture, draw, and unbind callbacks still remain retained in
   the legacy Canvas path.
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge()` temporary
+  erase and paint branch wiring now routes through
+  `execute_legacy_canvas_draw_merge_temporary_composite(...)`; the retained
+  path still owns the concrete setup, sampler, texture, draw, and unbind
+  callbacks.
 - 2026-06-13: DEBT-0036 was narrowed again. `NodeStrokePreview::draw_stroke_immediate()`
   now routes retained preview feedback/material/composite planning plus stroke
   shader uniform assembly through

docs/modernization/tasks.md

@@ -660,6 +660,11 @@ Progress Notes:
   path still owns the concrete setup, sampler, texture, draw, and unbind
   callbacks. Next slice should target another narrow draw-merge seam without
   reopening the landed temporary-composite helper.
+- 2026-06-13: `Canvas::draw_merge()` temporary erase and paint branch wiring
+  now routes through `execute_legacy_canvas_draw_merge_temporary_composite(...)`;
+  the retained path still owns the concrete setup, sampler, texture, draw, and
+  unbind callbacks. Next slice should target another narrow draw-merge seam
+  without reopening the landed temporary-composite helper.
 - 2026-06-13: `pp_paint_renderer_stroke_execution_tests` now also covers
   retained frame-plan assembly for previous-sample projection mode and zoom
   scaling. Next slice should target the remaining preview/Canvas stroke

src/canvas.cpp

@@ -480,22 +480,6 @@ void Canvas::stroke_draw_mix(const glm::vec2& bb_min, const glm::vec2& bb_sz)
     gl.restore();
 }
 
-template <typename Setup, typename BindSamplers, typename BindTextures, typename Draw, typename UnbindTextures>
-static void execute_canvas_draw_merge_temporary_composite(
-    Setup&& setup,
-    BindSamplers&& bind_samplers,
-    BindTextures&& bind_textures,
-    Draw&& draw,
-    UnbindTextures&& unbind_textures)
-{
-    pp::panopainter::execute_legacy_canvas_stroke_temporary_composite(
-        std::forward<Setup>(setup),
-        std::forward<BindSamplers>(bind_samplers),
-        std::forward<BindTextures>(bind_textures),
-        std::forward<Draw>(draw),
-        std::forward<UnbindTextures>(unbind_textures));
-}
-
 std::array<std::vector<vertex_t>, 6> Canvas::stroke_draw_project(std::array<vertex_t, 4>& B, bool project_3d /*= false*/, glm::mat4 mv /*= glm::mat4(1)*/) const
 {
     // intersect P with the current face to clip diverging points from the plane
@@ -1451,8 +1435,9 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
             use_blend,
             pp::panopainter::LegacyCanvasDrawMergeLayerCompositeExecution {
                 .execute_temporary_erase = [&] {
-                        execute_canvas_draw_merge_temporary_composite(
+                    pp::panopainter::execute_legacy_canvas_draw_merge_temporary_composite(
-                    [&] {
+                        pp::panopainter::LegacyCanvasDrawMergeTemporaryCompositeExecution {
+                            .setup = [&] {
                                 //ShaderManager::u_vec2(kShaderUniform::Resolution, zw(m_box) / zoom);
                                 //ShaderManager::u_int(kShaderUniform::Lock, m_layers[layer_index]->m_alpha_locked);
                                 pp::panopainter::setup_legacy_stroke_erase_shader(
@@ -1465,12 +1450,12 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                                         .mask_enabled = m_smask_active,
                                     });
                             },
-                    [&] {
+                            .bind_samplers = [&] {
                                 m_sampler.bind(0);
                                 m_sampler.bind(1);
                                 m_sampler.bind(2);
                             },
-                    [&] {
+                            .bind_textures = [&] {
                                 set_active_texture_unit(0);
                                 m_layers[layer_index]->rtt(plane_index).bindTexture();
                                 set_active_texture_unit(1);
@@ -1478,15 +1463,16 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                                 set_active_texture_unit(2);
                                 m_smask.rtt(plane_index).bindTexture();
                             },
-                    [&] {
+                            .draw = [&] {
                                 m_plane.draw_fill();
                             },
-                    [&] {
+                            .unbind_textures = [&] {
                                 m_smask.rtt(plane_index).unbindTexture();
                                 set_active_texture_unit(1);
                                 m_tmp[plane_index].unbindTexture();
                                 set_active_texture_unit(0);
                                 m_layers[layer_index]->rtt(plane_index).unbindTexture();
+                            },
                         });
                 },
                     .execute_temporary_paint = [&] {
@@ -1495,8 +1481,9 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                         if (b->m_pattern_flipx) patt_scale.x *= -1.f;
                         if (b->m_pattern_flipy) patt_scale.y *= -1.f;
 
-                        execute_canvas_draw_merge_temporary_composite(
+                        pp::panopainter::execute_legacy_canvas_draw_merge_temporary_composite(
-                    [&] {
+                            pp::panopainter::LegacyCanvasDrawMergeTemporaryCompositeExecution {
+                                .setup = [&] {
                                     pp::panopainter::setup_legacy_stroke_composite_shader(
                                         pp::panopainter::LegacyStrokeCompositeUniforms {
                                             .resolution = Canvas::I->m_size,
@@ -1521,14 +1508,14 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                                             .use_pattern = stroke_material.composite_pass.use_pattern,
                                         });
                                 },
-                    [&] {
+                                .bind_samplers = [&] {
                                     m_sampler.bind(0);
                                     m_sampler.bind(1);
                                     m_sampler.bind(2);
                                     m_sampler.bind(3);
                                     m_sampler_stencil.bind(4);
                                 },
-                    [&] {
+                                .bind_textures = [&] {
                                     set_active_texture_unit(0);
                                     m_layers[layer_index]->rtt(plane_index).bindTexture();
                                     set_active_texture_unit(1);
@@ -1543,10 +1530,10 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                                         b->m_pattern_texture->bind() :
                                         unbind_texture_2d();
                                 },
-                    [&] {
+                                .draw = [&] {
                                     m_plane.draw_fill();
                                 },
-                    [&] {
+                                .unbind_textures = [&] {
                                     set_active_texture_unit(3);
                                     if (stroke_material.composite_pass.use_dual)
                                         m_tmp_dual[plane_index].unbindTexture();
@@ -1556,6 +1543,7 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                                     m_tmp[plane_index].unbindTexture();
                                     set_active_texture_unit(0);
                                     m_layers[layer_index]->rtt(plane_index).unbindTexture();
+                                },
                             });
                     },
                     .execute_layer_texture = [&] {

src/legacy_canvas_draw_merge_services.h

@@ -94,6 +94,14 @@ struct LegacyCanvasDrawMergeLayerCompositeExecution {
     std::function<void()> execute_layer_blend;
 };
 
+struct LegacyCanvasDrawMergeTemporaryCompositeExecution {
+    std::function<void()> setup;
+    std::function<void()> bind_samplers;
+    std::function<void()> bind_textures;
+    std::function<void()> draw;
+    std::function<void()> unbind_textures;
+};
+
 struct LegacyCanvasDrawMergePlaneSetupUniforms {
     LegacyCanvasDrawMergeCheckerboardUniforms checkerboard;
     bool use_blend = false;
@@ -330,6 +338,16 @@ inline void execute_legacy_canvas_draw_merge_layer_composite(
     }
 }
 
+inline void execute_legacy_canvas_draw_merge_temporary_composite(
+    const LegacyCanvasDrawMergeTemporaryCompositeExecution& execution)
+{
+    execution.setup();
+    execution.bind_samplers();
+    execution.bind_textures();
+    execution.draw();
+    execution.unbind_textures();
+}
+
 inline void execute_legacy_canvas_draw_merge_plane_setup(
     const LegacyCanvasDrawMergePlaneSetupUniforms& uniforms,
     const LegacyCanvasDrawMergePlaneSetupExecution& execution)