Extract Canvas draw_merge plane setup helper

docs/modernization/debt.md

@@ -18,6 +18,11 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Recent Reductions
 
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge()` now routes
+  per-plane merge-target clear, blend-state gating, and optional checkerboard
+  prepass through `execute_legacy_canvas_draw_merge_plane_setup(...)`;
+  per-plane iteration, per-layer branch selection, and concrete framebuffer
+  ownership remain in the legacy Canvas path.
 - 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge()` now routes
   the standard per-layer texture-alpha pass through
   `execute_legacy_canvas_draw_merge_layer_texture(...)`; per-plane iteration,

docs/modernization/roadmap.md

@@ -67,6 +67,10 @@ Recent 2026-06-13 retained preview reductions continue to narrow DEBT-0036:
 feedback/material/composite planning and stroke-shader uniform assembly through
 `plan_legacy_node_stroke_preview_pass_orchestration(...)`, leaving the preview
 node with a smaller live-GL callback surface around pass execution.
+`Canvas::draw_merge()` now also routes its per-plane merge-target clear,
+blend-state gating, and optional checkerboard prepass through
+`execute_legacy_canvas_draw_merge_plane_setup(...)`, reducing another retained
+plane-setup branch.
 `Canvas::draw_merge()` also now routes its standard per-layer texture-alpha pass
 through `execute_legacy_canvas_draw_merge_layer_texture(...)`, reducing another
 piece of retained layer-composite sequencing.

docs/modernization/tasks.md

@@ -509,6 +509,13 @@ Done Checks:
 
 Progress Notes:
 
+- 2026-06-13: `Canvas::draw_merge()` now routes per-plane merge-target clear,
+  blend-state gating, and optional checkerboard prepass through
+  `execute_legacy_canvas_draw_merge_plane_setup(...)`; per-layer iteration,
+  temporary-stroke branches, and concrete framebuffer ownership remain local to
+  `Canvas`. Next slice should target another narrow draw-merge execution seam
+  without reopening landed plane-setup, temporary-composite, layer-blend,
+  final-plane, or texture-alpha helpers.
 - 2026-06-13: `Canvas::draw_merge()` now routes the standard per-layer
   texture-alpha pass through
   `execute_legacy_canvas_draw_merge_layer_texture(...)`; temporary-stroke

src/canvas.cpp

@@ -1371,26 +1371,29 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
             continue;
 
         m_layers_merge.rtt(plane_index).bindFramebuffer();
-        m_layers_merge.rtt(plane_index).clear({ 1, 1, 1, 0 });
-
-        if (use_blend)
-        {
-            apply_canvas_capability(blend_state(), false);
-            m_layers_merge.rtt(plane_index).clear();
-        }
-        else
-        {
-            if (draw_checkerboard)
-            {
-                pp::panopainter::setup_legacy_canvas_draw_merge_checkerboard_shader(
-                    pp::panopainter::LegacyCanvasDrawMergeCheckerboardUniforms {
-                        .mvp = ortho,
-                        .colorize = false,
-                    });
-                m_plane.draw_fill();
-            }
-            apply_canvas_capability(blend_state(), true);
-        }
+        pp::panopainter::execute_legacy_canvas_draw_merge_plane_setup(
+            pp::panopainter::LegacyCanvasDrawMergePlaneSetupUniforms {
+                .checkerboard = {
+                    .mvp = ortho,
+                    .colorize = false,
+                },
+                .use_blend = use_blend,
+                .draw_checkerboard = draw_checkerboard,
+            },
+            pp::panopainter::LegacyCanvasDrawMergePlaneSetupExecution {
+                .clear_plane = [&] {
+                    m_layers_merge.rtt(plane_index).clear({ 1, 1, 1, 0 });
+                },
+                .disable_blend = [&] {
+                    apply_canvas_capability(blend_state(), false);
+                },
+                .enable_blend = [&] {
+                    apply_canvas_capability(blend_state(), true);
+                },
+                .draw = [&] {
+                    m_plane.draw_fill();
+                },
+            });
 
         for (int layer_index = 0; layer_index < m_layers.size(); layer_index++)
         {

src/legacy_canvas_draw_merge_services.h

@@ -87,6 +87,19 @@ struct LegacyCanvasDrawMergeLayerTextureExecution {
     std::function<void()> unbind_layer_texture;
 };
 
+struct LegacyCanvasDrawMergePlaneSetupUniforms {
+    LegacyCanvasDrawMergeCheckerboardUniforms checkerboard;
+    bool use_blend = false;
+    bool draw_checkerboard = false;
+};
+
+struct LegacyCanvasDrawMergePlaneSetupExecution {
+    std::function<void()> clear_plane;
+    std::function<void()> disable_blend;
+    std::function<void()> enable_blend;
+    std::function<void()> draw;
+};
+
 struct LegacyCanvasDrawMergeFinalPlaneCompositeUniforms {
     LegacyCanvasDrawMergeCheckerboardUniforms checkerboard;
     LegacyCanvasDrawMergeTextureUniforms texture;
@@ -291,6 +304,26 @@ inline void execute_legacy_canvas_draw_merge_layer_texture(
     execution.unbind_layer_texture();
 }
 
+inline void execute_legacy_canvas_draw_merge_plane_setup(
+    const LegacyCanvasDrawMergePlaneSetupUniforms& uniforms,
+    const LegacyCanvasDrawMergePlaneSetupExecution& execution)
+{
+    execution.clear_plane();
+
+    if (uniforms.use_blend) {
+        execution.disable_blend();
+        execution.clear_plane();
+        return;
+    }
+
+    if (uniforms.draw_checkerboard) {
+        setup_legacy_canvas_draw_merge_checkerboard_shader(uniforms.checkerboard);
+        execution.draw();
+    }
+
+    execution.enable_blend();
+}
+
 inline void execute_legacy_canvas_draw_merge_final_plane_composite(
     const LegacyCanvasDrawMergeFinalPlaneCompositeUniforms& uniforms,
     const LegacyCanvasDrawMergeFinalPlaneCompositeExecution& execution)