Now layer start X/Y can be < 0 when machine centre is zero.

resources/definitions/fdmprinter.def.json

@@ -2997,7 +2997,7 @@
                     "unit": "mm",
                     "type": "float",
                     "default_value": 0.0,
-                    "minimum_value": "0",
+                    "minimum_value": "machine_width / -2 if machine_center_is_zero else 0",
                     "settable_per_mesh": false,
                     "settable_per_extruder": true,
                     "settable_per_meshgroup": true
@@ -3009,7 +3009,7 @@
                     "unit": "mm",
                     "type": "float",
                     "default_value": 0.0,
-                    "minimum_value": "0",
+                    "minimum_value": "machine_depth / -2 if machine_center_is_zero else 0",
                     "settable_per_mesh": false,
                     "settable_per_extruder": true,
                     "settable_per_meshgroup": true