Route stroke mixer composite setup through helper

docs/modernization/debt.md

@@ -18,6 +18,12 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Recent Reductions
 
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::stroke_draw_mix` now
+  reuses `legacy_canvas_stroke_composite_services.h` for mixer-pass retained
+  `kShader::CompDraw` binding and blend uniform writes, with the helper now
+  preserving caller-specific texture slot uniforms. Mixer framebuffer,
+  viewport/scissor/capability state, sampler binding, texture binding, and draw
+  execution remain retained Canvas code.
 - 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

docs/modernization/roadmap.md

@@ -2992,6 +2992,10 @@ Results:
   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::stroke_draw_mix` now shares the retained stroke composite shader
+  helper for mixer-pass `CompDraw` setup, while preserving its caller-specific
+  texture slot uniforms. Mixer framebuffer/scissor state, sampler and texture
+  binding, and draw execution remain retained.
 - 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

@@ -437,21 +437,19 @@ void Canvas::stroke_draw_mix(const glm::vec2& bb_min, const glm::vec2& bb_sz)
         m_sampler.bind(1);
         m_sampler.bind(2);
         const auto& b = m_current_stroke->m_brush;
-        ShaderManager::use(kShader::CompDraw);
+        pp::panopainter::setup_legacy_stroke_composite_shader(
-        ShaderManager::u_int(kShaderUniform::Tex, 0);
+            pp::panopainter::LegacyStrokeCompositeUniforms {
-        //ShaderManager::u_int(kShaderUniform::TexA, 0);
+                .resolution = m_size,
-        ShaderManager::u_int(kShaderUniform::TexStroke, 1);
+                .mvp = plane_mvp_z,
-        ShaderManager::u_int(kShaderUniform::TexMask, 2);
+                .pattern_texture_slot = 3,
-        ShaderManager::u_vec2(kShaderUniform::Resolution, m_size);
+                .layer_alpha = 1.0f,
-        ShaderManager::u_int(kShaderUniform::TexPattern, 3);
+                .alpha_lock = false, /*m_layers[layer_index]->m_alpha_locked*/
-        ShaderManager::u_float(kShaderUniform::Alpha, 1);
+                .mask_enabled = false, /*m_smask_active*/
-        ShaderManager::u_int(kShaderUniform::Lock, false/*m_layers[layer_index]->m_alpha_locked*/);
+                .use_fragcoord = false,
-        ShaderManager::u_int(kShaderUniform::Mask, false/*m_smask_active*/);
+                .blend_mode = b->m_blend_mode,
-        ShaderManager::u_int(kShaderUniform::UseFragcoord, false);
+                .use_dual = false,
-        ShaderManager::u_int(kShaderUniform::UseDual, false);
+                .use_pattern = false,
-        ShaderManager::u_int(kShaderUniform::UsePattern, false);
+            });
-        ShaderManager::u_int(kShaderUniform::BlendMode, b->m_blend_mode);
-        ShaderManager::u_mat4(kShaderUniform::MVP, plane_mvp_z);
         set_active_texture_unit(0);
         m_layers[layer_index]->rtt(plane_index).bindTexture();
         set_active_texture_unit(1);

src/legacy_canvas_stroke_composite_services.h

@@ -21,6 +21,11 @@ struct LegacyStrokeCompositeUniforms {
     glm::vec2 resolution {};
     LegacyStrokeCompositePatternUniforms pattern;
     glm::mat4 mvp { 1.0f };
+    int texture_slot = 0;
+    int stroke_texture_slot = 1;
+    int mask_texture_slot = 2;
+    int dual_texture_slot = 3;
+    int pattern_texture_slot = 4;
     float layer_alpha = 1.0f;
     bool alpha_lock = false;
     bool mask_enabled = false;
@@ -69,11 +74,11 @@ inline void setup_legacy_stroke_composite_shader(
     const LegacyStrokeCompositeShaderExecution& execution) noexcept
 {
     execution.use_shader(kShader::CompDraw);
-    execution.set_int(kShaderUniform::Tex, 0);
+    execution.set_int(kShaderUniform::Tex, uniforms.texture_slot);
-    execution.set_int(kShaderUniform::TexStroke, 1);
+    execution.set_int(kShaderUniform::TexStroke, uniforms.stroke_texture_slot);
-    execution.set_int(kShaderUniform::TexMask, 2);
+    execution.set_int(kShaderUniform::TexMask, uniforms.mask_texture_slot);
-    execution.set_int(kShaderUniform::TexDual, 3);
+    execution.set_int(kShaderUniform::TexDual, uniforms.dual_texture_slot);
-    execution.set_int(kShaderUniform::TexPattern, 4);
+    execution.set_int(kShaderUniform::TexPattern, uniforms.pattern_texture_slot);
     execution.set_vec2(kShaderUniform::Resolution, uniforms.resolution);
     execution.set_float(kShaderUniform::Alpha, uniforms.layer_alpha);
     execution.set_int(kShaderUniform::Lock, uniforms.alpha_lock);