Isolate draw merge background shader setup

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` checkerboard
+  background shader setup and final merged-texture redraw setup now route
+  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::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

@@ -2988,6 +2988,10 @@ Results:
   for its source-over-destination `CompDraw` uniform setup, while keeping the
   existing layer dirty mutation, framebuffer copy, texture binding, and draw
   order local to the Canvas layer-merge path.
+- `Canvas::draw_merge` now shares a retained draw-merge shader setup helper for
+  checkerboard backgrounds and the final merged texture redraw over the grid;
+  layer traversal, blend destination copies, sampler/texture binding, and draw
+  ordering remain in the legacy Canvas path.
 - Canvas thumbnail layer blending now uses the same canvas destination-feedback
   plan for framebuffer-fetch versus texture-copy decisions; the thumbnail draw
   itself still executes through retained OpenGL canvas code under DEBT-0036.

src/canvas.cpp

@@ -2,6 +2,7 @@
 #include "log.h"
 #include "canvas.h"
 #include "app.h"
+#include "legacy_canvas_draw_merge_services.h"
 #include "legacy_gl_renderbuffer_dispatch.h"
 #include "legacy_canvas_stroke_commit_services.h"
 #include "legacy_canvas_stroke_composite_services.h"
@@ -1273,9 +1274,11 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
         {
             if (draw_checkerboard)
             {
-                ShaderManager::use(kShader::Checkerboard);
+                pp::panopainter::setup_legacy_canvas_draw_merge_checkerboard_shader(
-                ShaderManager::u_int(kShaderUniform::Colorize, false);
+                    pp::panopainter::LegacyCanvasDrawMergeCheckerboardUniforms {
-                ShaderManager::u_mat4(kShaderUniform::MVP, ortho);
+                        .mvp = ortho,
+                        .colorize = false,
+                    });
                 m_plane.draw_fill();
             }
             apply_canvas_capability(blend_state(), true);
@@ -1454,9 +1457,11 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
             //draw the grid
             if (draw_checkerboard)
             {
-                ShaderManager::use(kShader::Checkerboard);
+                pp::panopainter::setup_legacy_canvas_draw_merge_checkerboard_shader(
-                ShaderManager::u_int(kShaderUniform::Colorize, false);
+                    pp::panopainter::LegacyCanvasDrawMergeCheckerboardUniforms {
-                ShaderManager::u_mat4(kShaderUniform::MVP, ortho);
+                        .mvp = ortho,
+                        .colorize = false,
+                    });
                 m_plane.draw_fill();
             }
 
@@ -1464,9 +1469,11 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
             m_sampler.bind(0);
             set_active_texture_unit(0);
             m_merge_tex.bind();
-            ShaderManager::use(kShader::Texture);
+            pp::panopainter::setup_legacy_canvas_draw_merge_texture_shader(
-            ShaderManager::u_int(kShaderUniform::Tex, 0);
+                pp::panopainter::LegacyCanvasDrawMergeTextureUniforms {
-            ShaderManager::u_mat4(kShaderUniform::MVP, ortho);
+                    .mvp = ortho,
+                    .texture_slot = 0,
+                });
             m_plane.draw_fill();
             m_merge_tex.unbind();
         }

src/legacy_canvas_draw_merge_services.h

@@ -0,0 +1,64 @@
+#pragma once
+
+#include "shader.h"
+
+#include <functional>
+
+namespace pp::panopainter {
+
+struct LegacyCanvasDrawMergeCheckerboardUniforms {
+    glm::mat4 mvp { 1.0f };
+    bool colorize = false;
+};
+
+struct LegacyCanvasDrawMergeTextureUniforms {
+    glm::mat4 mvp { 1.0f };
+    int texture_slot = 0;
+};
+
+struct LegacyCanvasDrawMergeShaderExecution {
+    std::function<void(kShader)> use_shader;
+    std::function<void(kShaderUniform, int)> set_int;
+    std::function<void(kShaderUniform, const glm::mat4&)> set_mat4;
+};
+
+[[nodiscard]] inline LegacyCanvasDrawMergeShaderExecution legacy_shader_manager_draw_merge_execution() noexcept
+{
+    return {
+        .use_shader = [](kShader shader) { ShaderManager::use(shader); },
+        .set_int = [](kShaderUniform uniform, int value) { ShaderManager::u_int(uniform, value); },
+        .set_mat4 = [](kShaderUniform uniform, const glm::mat4& value) { ShaderManager::u_mat4(uniform, value); },
+    };
+}
+
+inline void setup_legacy_canvas_draw_merge_checkerboard_shader(
+    const LegacyCanvasDrawMergeCheckerboardUniforms& uniforms,
+    const LegacyCanvasDrawMergeShaderExecution& execution) noexcept
+{
+    execution.use_shader(kShader::Checkerboard);
+    execution.set_int(kShaderUniform::Colorize, uniforms.colorize);
+    execution.set_mat4(kShaderUniform::MVP, uniforms.mvp);
+}
+
+inline void setup_legacy_canvas_draw_merge_checkerboard_shader(
+    const LegacyCanvasDrawMergeCheckerboardUniforms& uniforms)
+{
+    setup_legacy_canvas_draw_merge_checkerboard_shader(uniforms, legacy_shader_manager_draw_merge_execution());
+}
+
+inline void setup_legacy_canvas_draw_merge_texture_shader(
+    const LegacyCanvasDrawMergeTextureUniforms& uniforms,
+    const LegacyCanvasDrawMergeShaderExecution& execution) noexcept
+{
+    execution.use_shader(kShader::Texture);
+    execution.set_int(kShaderUniform::Tex, uniforms.texture_slot);
+    execution.set_mat4(kShaderUniform::MVP, uniforms.mvp);
+}
+
+inline void setup_legacy_canvas_draw_merge_texture_shader(
+    const LegacyCanvasDrawMergeTextureUniforms& uniforms)
+{
+    setup_legacy_canvas_draw_merge_texture_shader(uniforms, legacy_shader_manager_draw_merge_execution());
+}
+
+} // namespace pp::panopainter