Extract draw merge temporary paint branch

docs/modernization/debt.md

@@ -198,6 +198,11 @@ agent or engineer to remove them without reconstructing context from chat.
   `make_legacy_canvas_draw_merge_temporary_erase_composite(...)`; the live
   path still owns the concrete layer RTT, mixer RTT, sampler, and plane
   callbacks.
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge()` temporary
+  paint branch execution now routes through
+  `make_legacy_canvas_draw_merge_temporary_paint_composite(...)`; the live
+  path still owns the concrete layer RTT, mixer RTT, sampler, and plane
+  callbacks.
 - 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge()` now routes
   the layer-composite shell through a local wrapper around
   `execute_legacy_canvas_draw_merge_layer_composite(...)`; the final branch

src/canvas.cpp

@@ -1485,9 +1485,8 @@ 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;
 
-                        pp::panopainter::execute_legacy_canvas_draw_merge_temporary_composite(
+                        const auto execution = pp::panopainter::make_legacy_canvas_draw_merge_temporary_paint_composite(
-                            pp::panopainter::LegacyCanvasDrawMergeTemporaryCompositeExecution {
+                            [&] {
-                                .setup = [&] {
                                 pp::panopainter::setup_legacy_stroke_composite_shader(
                                     pp::panopainter::LegacyStrokeCompositeUniforms {
                                         .resolution = Canvas::I->m_size,
@@ -1512,14 +1511,14 @@ 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);
@@ -1534,10 +1533,10 @@ 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();
@@ -1547,8 +1546,8 @@ 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();
-                                },
                             });
+                        pp::panopainter::execute_legacy_canvas_draw_merge_temporary_composite(execution);
                     },
                     .execute_layer_texture = [&] {
                         execute_canvas_draw_merge_layer_texture(

src/legacy_canvas_draw_merge_services.h

@@ -124,6 +124,28 @@ template <
     };
 }
 
+template <
+    typename Setup,
+    typename BindSamplers,
+    typename BindTextures,
+    typename Draw,
+    typename UnbindTextures>
+[[nodiscard]] inline LegacyCanvasDrawMergeTemporaryCompositeExecution make_legacy_canvas_draw_merge_temporary_paint_composite(
+    Setup&& setup,
+    BindSamplers&& bind_samplers,
+    BindTextures&& bind_textures,
+    Draw&& draw,
+    UnbindTextures&& unbind_textures)
+{
+    return LegacyCanvasDrawMergeTemporaryCompositeExecution {
+        .setup = std::forward<Setup>(setup),
+        .bind_samplers = std::forward<BindSamplers>(bind_samplers),
+        .bind_textures = std::forward<BindTextures>(bind_textures),
+        .draw = std::forward<Draw>(draw),
+        .unbind_textures = std::forward<UnbindTextures>(unbind_textures),
+    };
+}
+
 struct LegacyCanvasDrawMergePlaneSetupUniforms {
     LegacyCanvasDrawMergeCheckerboardUniforms checkerboard;
     bool use_blend = false;

tests/paint_renderer/compositor_tests.cpp

@@ -3194,6 +3194,28 @@ void legacy_canvas_draw_merge_temporary_erase_helper_preserves_order(pp::tests::
     PP_EXPECT(h, order == expected);
 }
 
+void legacy_canvas_draw_merge_temporary_paint_helper_preserves_order(pp::tests::Harness& h)
+{
+    std::vector<std::string> order;
+    const auto execution = pp::panopainter::make_legacy_canvas_draw_merge_temporary_paint_composite(
+        [&] { order.emplace_back("setup"); },
+        [&] { order.emplace_back("bind_samplers"); },
+        [&] { order.emplace_back("bind_textures"); },
+        [&] { order.emplace_back("draw"); },
+        [&] { order.emplace_back("unbind_textures"); });
+
+    pp::panopainter::execute_legacy_canvas_draw_merge_temporary_composite(execution);
+
+    const std::vector<std::string> expected {
+        "setup",
+        "bind_samplers",
+        "bind_textures",
+        "draw",
+        "unbind_textures",
+    };
+    PP_EXPECT(h, order == expected);
+}
+
 void plans_canvas_stroke_feedback_paths(pp::tests::Harness& h)
 {
     const Extent2D extent { .width = 32, .height = 16 };
@@ -3557,6 +3579,9 @@ int main()
     harness.run(
         "legacy_canvas_draw_merge_temporary_erase_helper_preserves_order",
         legacy_canvas_draw_merge_temporary_erase_helper_preserves_order);
+    harness.run(
+        "legacy_canvas_draw_merge_temporary_paint_helper_preserves_order",
+        legacy_canvas_draw_merge_temporary_paint_helper_preserves_order);
     harness.run("plans_canvas_stroke_feedback_paths", plans_canvas_stroke_feedback_paths);
     harness.run("canvas_stroke_feedback_preserves_legacy_fallback", canvas_stroke_feedback_preserves_legacy_fallback);
     harness.run("plans_canvas_stroke_rasterization_boundary", plans_canvas_stroke_rasterization_boundary);