Belt: check object height against the gantry clearance again

validate() skipped the build-volume height check whenever the machine-frame
transform was active, which is every shipped belt profile, so a 400 mm
object passed on a 300 mm printable_height. The transform only changes how
the height is written to G-code; the clearance check from f682ab5cd3
applies regardless.

Raised in Hanif Koh's review of #14394.

src/libslic3r/Print.cpp

@@ -20,7 +20,6 @@
 #include "GCode.hpp"
 #include "BeltGCode.hpp"
 #include "BeltTransform.hpp"
-#include "GCode/MachineFrameTransform.hpp"
 #include "GCode/WipeTower.hpp"
 #include "GCode/WipeTower2.hpp"
 #include "GCode/WipeTowerEstimate.hpp"
@@ -1621,17 +1620,11 @@ StringObjectException Print::validate(std::vector<StringObjectException> *warnin
     // is not comparable to printable_height (which is gantry clearance in the
     // build-volume frame).  Compare against the model's pre-shear Z instead,
     // mirroring the bbox computed in PrintObject::update_slicing_parameters.
-    // When the post-gcode MachineFrameTransform is active the printer's
-    // physical Z mapping is non-trivial — skip the check entirely.
+    // The machine-frame transform only changes how that height is written to
+    // G-code, not how much room there is under the gantry.
     const bool belt_printer = this->config().belt_printer.value;
-    bool skip_max_height_check = false;
-    if (belt_printer) {
-        MachineFrameTransform machine_frame;
-        machine_frame.init_from_config(this->config());
-        skip_max_height_check = machine_frame.is_active();
-    }
     const double shrinkage_compensation_z = this->shrinkage_compensation().z();
-    for (size_t print_object_idx = 0; !skip_max_height_check && print_object_idx < m_objects.size(); ++ print_object_idx) {
+    for (size_t print_object_idx = 0; print_object_idx < m_objects.size(); ++ print_object_idx) {
         const PrintObject &print_object = *m_objects[print_object_idx];
 
         double effective_max_z       = 0;

tests/fff_print/test_print.cpp

@@ -562,3 +562,37 @@ TEST_CASE("Belt printers never start a scarf seam below the layer", "[Print][bel
     });
     CHECK(worst_backstep < 0.2);
 }
+
+// printable_height on a belt printer is the clearance under the gantry, so an object taller
+// than that is refused whatever the machine-frame transform does to the emitted coordinates.
+TEST_CASE("Belt printers refuse an object taller than the gantry clearance", "[Print][belt]")
+{
+    auto belt_config = [](double printable_height) {
+        DynamicPrintConfig config = DynamicPrintConfig::full_print_config();
+        config.set_deserialize_strict({
+            { "belt_printer",               1 },
+            { "belt_slice_rotation",        "x" },
+            { "belt_slice_rotation_angle",  45 },
+            { "belt_slice_rotation_global", 1 },
+            { "gcode_remap_x",              "rev_x" },
+            { "gcode_remap_y",              "pos_z" },
+            { "gcode_remap_z",              "pos_y" },
+            { "printable_height",           printable_height },
+            { "skirt_loops",                0 },
+        });
+        return config;
+    };
+
+    SECTION("a 20 mm cube fits under 50 mm of clearance") {
+        Print print;
+        Model model;
+        init_print({ cube(20) }, print, model, belt_config(50));
+        CHECK(print.validate().string.empty());
+    }
+    SECTION("a 60 mm cube does not") {
+        Print print;
+        Model model;
+        init_print({ cube(60) }, print, model, belt_config(50));
+        CHECK_FALSE(print.validate().string.empty());
+    }
+}