Let Cancel stop a texture bake while it simplifies

Once the budget is met the progress fraction stops moving, and a cancel was only checked when it
moved, so Cancel did nothing until the flat-face merging finished. It is now also checked every 16k
steps.

src/libslic3r/TextureBake/TextureBakeDecimate.cpp

@@ -457,6 +457,16 @@ DecimateResult decimate(const TriSoup &geometry, size_t target_triangles, bool h
 
         const HeapEntry top = heap_pop();
         ++pops;
+        if (on_progress) {
+            // Every 16 k pops as well as whenever the fraction moves: this is the only place a cancel
+            // is seen, and past the target the fraction stops moving.
+            const double p = std::min(1.0, double(init_faces - active_faces) / double(to_remove));
+            if (p - last_progress > 0.005 || (pops & 0x3fff) == 0) {
+                last_progress = p;
+                if (!on_progress(p))
+                    break;
+            }
+        }
         // The popped entry is the cheapest left, so exceeding the tolerance ends the run.
         if (reached_target && top.cost > harvest_ceil)
             break;
@@ -527,15 +537,6 @@ DecimateResult decimate(const TriSoup &geometry, size_t target_triangles, bool h
             }
         }
         maybe_compact();
-
-        if (on_progress) {
-            const double p = std::min(1.0, double(init_faces - active_faces) / double(to_remove));
-            if (p - last_progress > 0.005) {
-                last_progress = p;
-                if (!on_progress(p))
-                    break;
-            }
-        }
     }
 
     BOOST_LOG_TRIVIAL(info) << "TextureBake decimate: pops=" << pops << " stale=" << stale_pops

tests/libslic3r/test_texture_displacement.cpp

@@ -2032,3 +2032,25 @@ TEST_CASE("A budget met only by flattening curvature removes detail", "[TextureD
     CHECK(res.geometry.triangle_count() <= 100);
     CHECK(res.target_cost_detail);
 }
+
+TEST_CASE("Cancelling while flat faces are harvested past the budget stops the decimation", "[TextureDisplacement]")
+{
+    // 12 * 4^6 = 49152 triangles: the budget is met almost at once, and the harvest after it runs to
+    // a dozen triangles over several times the two polling intervals a cancel takes to be seen.
+    const TextureBake::TriSoup cube   = TextureBake::to_soup(subdivide_mesh_uniform(its_make_cube(20., 20., 20.), 0.f, 6));
+    const size_t               target = cube.triangle_count() - 1000;
+    const size_t full = TextureBake::decimate(cube, target, true, 0.005).geometry.triangle_count();
+
+    // The Cancel button pressed once the bar is full: every call after the first to report 1.0 cancels.
+    bool       full_seen = false;
+    const auto cancel_when_full = [&full_seen](double f) {
+        if (full_seen)
+            return false;
+        full_seen = f >= 1.0;
+        return true;
+    };
+    const size_t canceled = TextureBake::decimate(cube, target, true, 0.005, {}, cancel_when_full).geometry.triangle_count();
+    CHECK(full_seen);
+    CHECK(canceled <= target);
+    CHECK(canceled > full);
+}