Share retained erase temporary composite 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()` erase live
+  temporary-stroke composite now routes retained setup, sampler bind, texture
+  bind, draw, and texture unbind ordering through
+  `execute_legacy_canvas_stroke_temporary_composite(...)`; broader final
+  composite ownership remains in the legacy Canvas path.
 - 2026-06-13: DEBT-0036 was narrowed again. `NodeStrokePreview::stroke_draw_mix()`
   now routes retained mix-pass shader setup plus framebuffer/state/input/draw
   ordering through `execute_legacy_node_stroke_preview_mix_pass(...)`; the

docs/modernization/roadmap.md

@@ -3141,6 +3141,10 @@ Results:
   `execute_legacy_canvas_stroke_temporary_composite(...)` for setup, sampler
   bind, texture bind, draw, and texture unbind ordering, while erase-path and
   broader final composite ownership remain in the legacy Canvas path.
+- `Canvas::draw_merge()` erase live temporary-stroke composite now also shares
+  `execute_legacy_canvas_stroke_temporary_composite(...)`, leaving only the
+  concrete GL object callbacks and broader final composite ownership in the
+  legacy Canvas path.
 - `NodeStrokePreview::draw_stroke_immediate()` now shares
   `execute_legacy_node_stroke_preview_pass_sequence(...)` for
   dual-pass/background/main-pass/final-composite/copy-back ordering, while the

docs/modernization/tasks.md

@@ -509,6 +509,14 @@ Done Checks:
 
 Progress Notes:
 
+- 2026-06-13: `Canvas::draw_merge()` erase live temporary-stroke composite now
+  routes retained setup, sampler bind, texture bind, draw, and texture unbind
+  ordering through `execute_legacy_canvas_stroke_temporary_composite(...)`;
+  both live temporary composite branches now share the same retained execution
+  helper while `Canvas` keeps only concrete GL object callbacks and broader
+  final composite ownership. Next slice should target the remaining final
+  composite seam without reopening landed sample, mix, dirty, or framebuffer
+  helpers.
 - 2026-06-13: `NodeStrokePreview::stroke_draw_mix()` now routes retained
   mix-pass shader setup plus framebuffer/state/input/draw ordering through
   `execute_legacy_node_stroke_preview_mix_pass(...)`, with compositor coverage

src/canvas.cpp

@@ -1408,10 +1408,8 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
 
             if (m_current_stroke && m_current_mode == kCanvasMode::Erase && m_show_tmp && m_current_layer_idx == layer_index)
             {
-                m_sampler.bind(0);
+                pp::panopainter::execute_legacy_canvas_stroke_temporary_composite(
-                m_sampler.bind(1);
+                    [&] {
-                m_sampler.bind(2);
-
                         //ShaderManager::u_vec2(kShaderUniform::Resolution, zw(m_box) / zoom);
                         //ShaderManager::u_int(kShaderUniform::Lock, m_layers[layer_index]->m_alpha_locked);
                         pp::panopainter::setup_legacy_stroke_erase_shader(
@@ -1423,18 +1421,30 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                                 .alpha = m_layers[layer_index]->m_opacity,
                                 .mask_enabled = m_smask_active,
                             });
+                    },
+                    [&] {
+                        m_sampler.bind(0);
+                        m_sampler.bind(1);
+                        m_sampler.bind(2);
+                    },
+                    [&] {
                         set_active_texture_unit(0);
                         m_layers[layer_index]->rtt(plane_index).bindTexture();
                         set_active_texture_unit(1);
                         m_tmp[plane_index].bindTexture();
                         set_active_texture_unit(2);
                         m_smask.rtt(plane_index).bindTexture();
+                    },
+                    [&] {
                         m_plane.draw_fill();
+                    },
+                    [&] {
                         m_smask.rtt(plane_index).unbindTexture();
                         set_active_texture_unit(1);
                         m_tmp[plane_index].unbindTexture();
                         set_active_texture_unit(0);
                         m_layers[layer_index]->rtt(plane_index).unbindTexture();
+                    });
             }
             else if (m_current_stroke && m_show_tmp && m_current_layer_idx == layer_index)
             {