Extract stroke mix setup shell

docs/modernization/debt.md

@@ -18,6 +18,10 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Recent Reductions
 
+- 2026-06-13: `LATER-003` was narrowed again. `Canvas::stroke_draw_mix()` now
+  routes the setup/end shell through
+  `make_legacy_canvas_stroke_mix_pass_setup(...)`; the live path still owns
+  the concrete framebuffer bind and GL capability toggles.
 - 2026-06-13: `LATER-003` was narrowed again. `Canvas::stroke_draw_mix()` now
   routes its retained mix-pass execution shell through
   `execute_legacy_canvas_stroke_mix_pass_with_setup(...)` with a concrete

docs/modernization/tasks.md

@@ -595,6 +595,9 @@ Progress Notes:
   execution shell through `execute_legacy_canvas_stroke_mix_pass_with_setup(...)`
   with a concrete request builder; the live path still owns the OpenGL setup
   and per-plane texture wiring.
+- 2026-06-13: `Canvas::stroke_draw_mix()` now routes the setup/end shell through
+  `make_legacy_canvas_stroke_mix_pass_setup(...)`; the live path still owns the
+  concrete framebuffer bind and GL capability toggles.
 - 2026-06-13: `Canvas::stroke_draw()` live-pass sampler wiring now reuses a
   retained helper builder, and the stroke execution tests cover it. `STR-004`
   is now done after the final pad-destination helper extraction and tracker

src/canvas.cpp

@@ -403,8 +403,7 @@ void Canvas::stroke_draw_mix(const glm::vec2& bb_min, const glm::vec2& bb_sz)
             return current_layer.face(plane_index);
         });
     const auto& b = m_current_stroke->m_brush;
-    [[maybe_unused]] const auto mix_result =
+    const auto mix_setup = pp::panopainter::make_legacy_canvas_stroke_mix_pass_setup(
-        pp::panopainter::execute_legacy_canvas_stroke_mix_pass_with_setup(
         [&] {
                 m_mixer.bindFramebuffer();
                 apply_canvas_viewport(0, 0, m_mixer.getWidth(), m_mixer.getHeight());
@@ -419,7 +418,11 @@ void Canvas::stroke_draw_mix(const glm::vec2& bb_min, const glm::vec2& bb_sz)
             },
         [&] {
                 m_mixer.unbindFramebuffer();
-            },
+            });
+    [[maybe_unused]] const auto mix_result =
+        pp::panopainter::execute_legacy_canvas_stroke_mix_pass_with_setup(
+            mix_setup.begin,
+            mix_setup.end,
         pp::panopainter::make_legacy_canvas_stroke_mix_pass_request(
             "Canvas::stroke_draw_mix",
             m_size,

src/legacy_canvas_stroke_execution_services.h

@@ -340,6 +340,21 @@ struct LegacyCanvasStrokeMixPassResult {
     std::size_t composed_planes = 0;
 };
 
+struct LegacyCanvasStrokeMixPassSetup {
+    std::function<void()> begin;
+    std::function<void()> end;
+};
+
+[[nodiscard]] inline LegacyCanvasStrokeMixPassSetup make_legacy_canvas_stroke_mix_pass_setup(
+    std::function<void()> begin,
+    std::function<void()> end)
+{
+    return LegacyCanvasStrokeMixPassSetup {
+        .begin = std::move(begin),
+        .end = std::move(end),
+    };
+}
+
 [[nodiscard]] inline LegacyCanvasStrokeMixPassRequest make_legacy_canvas_stroke_mix_pass_request(
     std::string_view context,
     glm::vec2 resolution,

tests/paint_renderer/stroke_execution_tests.cpp

@@ -1885,6 +1885,24 @@ void retained_stroke_mix_pass_request_builder_preserves_callback_wiring(pp::test
     PP_EXPECT(h, events == expected_events);
 }
 
+void retained_stroke_mix_pass_setup_builder_preserves_boundaries(pp::tests::Harness& h)
+{
+    std::vector<std::string> events;
+    const auto setup = pp::panopainter::make_legacy_canvas_stroke_mix_pass_setup(
+        [&] { events.emplace_back("begin"); },
+        [&] { events.emplace_back("end"); });
+
+    PP_EXPECT(h, setup.begin);
+    PP_EXPECT(h, setup.end);
+
+    setup.begin();
+    setup.end();
+
+    PP_EXPECT(h, events.size() == 2U);
+    PP_EXPECT(h, events[0] == "begin");
+    PP_EXPECT(h, events[1] == "end");
+}
+
 } // namespace
 
 int main()
@@ -1992,5 +2010,8 @@ int main()
     harness.run(
         "retained_stroke_mix_pass_request_builder_preserves_callback_wiring",
         retained_stroke_mix_pass_request_builder_preserves_callback_wiring);
+    harness.run(
+        "retained_stroke_mix_pass_setup_builder_preserves_boundaries",
+        retained_stroke_mix_pass_setup_builder_preserves_boundaries);
     return harness.finish();
 }